Process for forming polarizer plate

ABSTRACT

The present invention generally relates to polymer films and a process for forming a polarizer plate comprising supplying one or more cover sheets on carrier webs, peeling the cover sheet from the carrier webs, spreading the cover sheets, and laminating the cover sheets to a polarizing film. The process may further comprise tension control, static dissipation after peeling the cover sheet from the carrier web, and an accumulator after a double splicing operation for splicing expiring web to fresh web.

FIELD OF THE INVENTION

The present invention generally relates to a process for making apolarizer plate. In particular, the process comprises supplying aprotective cover sheet on a carrier web and then peeling the cover sheetfrom the carrier web before laminating the cover sheet to a polarizingfilm.

BACKGROUND OF THE INVENTION

Transparent “resin films” are used in a variety of optical applications.For example, resin films are used in protective cover sheets for lightpolarizers in a variety of electronic displays, including Liquid CrystalDisplays (LCD).

The structure of LCDs may include a liquid crystal cell, one or morepolarizer plates, and one or more light management films. Liquid crystalcells are formed by confining liquid crystals such as vertically-aligned(VA), in-plane switching (IPS), twisted nematic (TN) or super twistednematic (STN) materials between two electrode substrates. Polarizerplates are typically a multi-layer element comprising resin films. Inparticular, a polarizer plate can comprise a polarizing film sandwichedbetween two protective cover sheets. Polarizing films are normallyprepared from a transparent and highly uniform, amorphous resin filmthat is subsequently stretched to orient the polymer molecules and thenstained with a dye to produce a dichroic film. An example of a suitableresin for the formation of polarizer films is fully hydrolyzed polyvinylalcohol (PVA). Because the stretched PVA films used to form polarizersare very fragile and dimensionally unstable, protective cover sheets arenormally laminated to both sides of the polarizing film to offer bothsupport and abrasion resistance.

Protective cover sheets of polarizer plates are required to have highuniformity, good dimensional and chemical stability, and hightransparency. Originally, protective coversheets were formed from glass,but a number of resin films are now used to produce lightweight andflexible polarizers. Many resins have been suggested for use inprotective cover sheets including cellulosics, acrylics, cyclic olefinpolymers, polycarbonates, and sulfones. However, acetyl cellulosepolymers are most commonly used in protective cover sheets for polarizerplates. Polymers of the acetyl cellulose type are commercially availablein a variety of molecular weights as well as the degree of acylsubstitution of the hydroxyl groups on the cellulose backbone. Of these,the fully substituted polymer, triacetyl cellulose (TAC) is commonlyused to manufacture resin films for use in protective cover sheets forpolarizer plates.

The cover sheet normally requires a surface treatment to insure goodadhesion to the PVA-dichroic film. When TAC is used as the protectivecover film of a polarizer plate, the TAC film is subjected to treatmentin an alkali bath to saponify the TAC surface to provide suitableadhesion to the PVA dichroic film. The alkali treatment uses an aqueoussolution containing a hydroxide of an alkali metal, such as sodiumhydroxide or potassium hydroxide. After alkali treatment, the celluloseacetate film is typically washed with weak acid solution followed byrinsing with water and drying. This saponification process is both messyand time consuming.

U.S. Pat. No. 2,362,580 describes a laminar structure wherein twocellulose ester films each having a surface layer containing cellulosenitrate and a modified PVA is adhered to both sides of a PVA film. JP06094915A discloses a protective film for polarizer plates wherein theprotective film has a hydrophilic layer which provides adhesion to PVAfilm. Commonly-assigned, copending U.S. patent application Ser. No.11/028,036 describes a guarded protective cover sheet having aremovable, carrier substrate and a cover sheet comprising a lowbirefringence protective polymer film and a layer promoting adhesion topolyvinyl alcohol on the same side of the carrier substrate as the lowbirefringence protective polymer film which eliminates the need for thesaponification process.

Protective cover sheets may be a composite or multilayer film includingother functional layers (herein also referred to as auxiliary layers)such as an antiglare layer, antireflection layer, anti-smudge layer,compensation layer, or antistatic layer. Generally, these functionallayers are applied in a process step that is separate from themanufacture of the low-birefringence protective polymer film, but may belater applied to a polarizing film as a composite film. An auxiliaryfilm may combine functions of more than one functional layer or aprotective polymer film may also serve the function of an auxiliarylayer.

For example, some LCD devices may contain a protective cover sheet thatalso serves as a compensation film to improve the viewing angle of animage. Compensation films (i.e. retardation films or phase differencefilms) are normally prepared from amorphous films that have a controlledlevel of birefringence prepared, for example, either by uniaxialstretching or by coating with discotic dyes. Suitable resins suggestedfor formation of compensation films by stretching include polyvinylalcohols, polycarbonates and sulfones. Compensation films prepared bytreatment with dyes normally require highly transparent films having lowbirefringence such as TAC and cyclic olefin polymers.

In general, resin films as described above are prepared either by meltextrusion methods or by casting methods. Melt extrusion methods involveheating the resin until molten (approximate viscosity on the order of100,000 cp), then applying the hot molten polymer to a highly polishedmetal band or drum with an extrusion die, cooling the film, and finallypeeling the film from the metal support. For several reasons, however,films prepared by melt extrusion are generally not suitable for opticalapplications. Principal among these is the fact that melt extruded filmsexhibit a high degree of optical birefringence. In the case of highlysubstituted cellulose acetate, there is the additional problem ofmelting the polymer. Cellulose triacetate has a very high meltingtemperature of 270-300° C., and this is above the temperature wheredecomposition begins. Films have been formed by melt extrusion at lowertemperatures by compounding cellulose acetate with various plasticizersas taught in U.S. Pat. No. 5,219,510 to Machell. However, the polymersdescribed in U.S. Pat. No. 5,219,510 to Machell are not the fullysubstituted cellulose triacetate, but rather have a lesser degree ofalkyl substitution or have propionate groups in place of some acetategroups. Even so, melt extruded films of cellulose acetate are known toexhibit poor flatness as noted in U.S. Pat. No. 5,753,140 to Shigenmura.For these reasons, melt extrusion methods are generally not practicalfor fabricating many resin films including cellulose triacetate filmsused to prepare protective covers and substrates in electronic displays.Rather, casting methods are generally preferred to manufacture thesefilms.

Resin films for optical applications are manufactured almost exclusivelyby casting methods. Casting methods involve first dissolving the polymerin an appropriate solvent to form a dope having a high viscosity on theorder of 50,000 cp, and then applying the viscous dope to a continuoushighly polished metal band or drum through an extrusion die, partiallydrying the wet film, peeling the partially dried film from the metalsupport, and conveying the partially dried film through an oven to morecompletely remove solvent from the film. Cast films typically have afinal dry thickness in the range of 40-200 microns. In general, thinfilms of less than 40 microns are very difficult to produce by castingmethods due to the fragility of wet film during the peeling and dryingprocesses. Films having a thickness of greater than 200 microns are alsoproblematic to manufacture due to difficulties associated with theremoval of solvent in the final drying step. Although the dissolutionand drying steps of the casting method add complexity and expense, castfilms generally have better optical properties when compared to filmsprepared by melt extrusion methods and, moreover, problems related todecomposition associated with exposure to high temperature are avoided.

Examples of optical films prepared by casting methods include: 1)Cellulose acetate sheets used to prepare light polarizing films asdisclosed in U.S. Pat. No. 4,895,769 to Land and U.S. Pat. No. 5,925,289to Cael as well as more recent disclosures in U.S. Patent Application.2001/0039319 A1 to Harita and U.S. Patent Application 2002/001700 A1 toSanefuji; 2) Cellulose triacetate sheets used for protective covers forlight polarizing films as disclosed in U.S. Pat. No. 5,695,694 to Iwata;3) Polycarbonate sheets used for protective covers for light polarizingfilms or for retardation plates as disclosed in U.S. Pat. No. 5,818,559to Yoshida and U.S. Pat. Nos. 5,478,518 and 5,561,180 both to Taketani;and (4) Polyethersulfone sheets used for protective covers for lightpolarizing films or for retardation plates as disclosed in U.S. Pat.Nos. 5,759,449 and 5,958,305 both to Shiro.

Commonly-assigned U.S. Patent Application Publications 2003/0215658A,2003/0215621A, 2003/0215608A, 2003/0215583A, 2003/0215582A,2003/0215581A, and 2003/0214715A describe a coating method to prepareresin films having low birefringence that are suitable for opticalapplications. The resin films are applied onto a discontinuous,removable carrier substrate from lower viscosity polymer solutions thanare normally used to prepare cast films. The dried film/substratecomposite is wound into rolls. U.S. 2003/0215608 A1 to Bermel statesthat a minimum level of adhesion between the film at the carriersubstrate is needed to avoid blister defects in a multi-pass film.However, excessive adhesion is undesirable since during subsequentpeeling operations the film may be damaged.

For optical films, good dimensional stability is necessary duringstorage as well as during subsequent fabrication of polarizer plates. Inaddition, resin films used in protective cover sheets for polarizerplates are susceptible to scratch and abrasion, as well as theaccumulation of dirt and dust, during the manufacture and handling ofthe cover sheet.

The preparation of high quality polarizer plates for displayapplications requires that the protective cover sheet be free of defectsdue to physical damage or the deposition of dirt and dust. It would bevery advantageous to avoid the need for saponification of cover sheetsin which the preparation of polarizer plates from resin films requires alamination process involving pretreatment in an alkali bath and thenapplication of adhesives, pressure, and high temperatures. Avoiding sucha saponification would improve both productivity and reduce thenecessary conveyance and handling of the sheets. Although advantageousfor cover sheets in general, this would be especially desirable forthinner cover sheets.

The preparation of very high quality polarizer plates would requireavoiding the various problems and defects known in the prior art, whichwould tend to be exacerbated when employing thinner protective coversheets. Such problems and defects include moving separation line,chatterlines, drawlines, sticky spots, creases, and web breaks.

SUMMARY OF THE INVENTION

The present invention relates generally to a method of making apolarizer plate involving peeling of a protective cover sheet from acarrier web prior to lamination to a polarizing film.

It is an object to provide an improved process for the fabrication ofpolarizer plates.

It is a further object of the present invention to overcome thelimitations of prior-art manufacture of polarizer plates and to providean improved method that eliminates the need for complex surfacetreatments such as saponification prior to the fabrication of polarizerplates.

It is another object to provide an improved process in which theprotective cover sheet is less susceptible to physical damage such asscratch and abrasion during the handling and processing steps necessaryin the fabrication of polarizer plates.

These and other objects of the invention are accomplished by a method inwhich the protective cover sheet for polarizers comprises a lowbirefringence protective polymer film and a layer promoting adhesion topolyvinyl alcohol films comprising a hydrophilic polymer, which coversheet is supplied on a carrier web.

The process provides excellent adhesion of a protective cover sheet tothe polyvinyl alcohol-containing dichroic polarizing films andeliminates the need to alkali treat the cover sheets prior to laminationto the dichroic films, thereby simplifying the process for manufacturingpolarizer plates.

Optionally, auxiliary layers that include an abrasion-resistant layer,antiglare layer, low reflection layer, antireflection layer, antistaticlayer, viewing angle compensation layer, and moisture barrier layer maybe employed in the cover sheets used in the process of the invention.

In particular, the present process comprises supplying at least one,preferably two, cover sheets on carrier webs, peeling the cover sheetsfrom the carrier web, and laminating the cover sheet to the PVA-dichroicfilm. The term “sheet” as used here, unless otherwise indicated, canrefer to a web that is unwound from or wound on a roll or the like. Theprocess may further comprise means for improved tension control, staticdissipation after peeling the cover sheet from the carrier web. Thecomposite comprising the cover sheet and carrier substrate arepreferably wound into rolls and stored until needed for the fabricationof polarizer plates.

More particularly, the present invention relates to a method of forminga polarizing plate comprising providing at least one guarded cover sheetcomposite comprising a carrier substrate and a cover sheet comprising alayer promoting adhesion to polyvinyl alcohol and a low birefringencepolymer film, providing a dichroic film, and bringing said cover sheetinto contact with said dichroic film such that the layer promotingadhesion to polyvinyl alcohol in said cover sheet is in contact withsaid dichroic film thereby producing a composite polarizer sheetcomprising a protective cover sheet and a dichroic film adhesivelyjoined by the adhesive layer, wherein said carrier substrate is peeledfrom the cover sheet prior to bringing the cover sheet into contact withthe dichroic film, and wherein the method further comprises conveying atleast one of the cover sheet, guarded cover sheet composite, andpolarizer sheet through an accumulator to allow continuous production ofthe composite polarizer sheet.

In one preferred embodiment of the present invention, a method offorming a polarizer plate comprises the following steps:

(a) supplying a first and second web (second web optional) from a firstand second unwinding spindle, respectively, each of said first andsecond webs comprising a guarded cover sheet having a carrier substrateand a protective cover sheet (plus optional auxiliary layers), theprotective cover sheet comprising a low birefringence polymer protectivefilm and a layer promoting adhesion to polyvinyl alcohol films;

(b) conveying each web in proximity to a means for double-sidedsplicing, wherein when each of said first or second webs is near toexpiring, which can occur independently, each web is secured to saidmeans for double splicing and then each such expiring web is doublespliced, preferably butt sliced, with a fresh web such that peeling andlaminating steps to follow are maintained in continuous operation;

(c) optionally conveying each web through an accumulator positionedbetween a first and second means for isolating tension in theaccumulator during splicing;

(d) conveying each web through the second means for isolating tensionwhich may be a drive;

(e) for each web, removing said carrier substrate from said protectivecover sheet at a peeling station, to produce at the point (incross-section) of peeling, respectively, (i) a first and secondunguarded web, each comprising the protective cover sheet, and (ii) afirst and second carrier web each comprising the carrier substrate;

(f) conveying each unguarded web over means for controlling tension, forexample, a load cell roller or float roller, having feedback control toa means for isolating tension (or a drive which may be the unwindingspindle or unwinder) located before the peeling station;

(g) preferably conveying each unguarded web over a means for spreadingthe web;

(h) bringing each unguarded web, either simultaneously or sequentially,into contact with a polarizing web comprising a dichroic PVA film suchthat each layer promoting adhesion to polyvinyl alcohol, in each of saidtwo unguarded webs, are contacted with said dichroic PVA film, whereinpressure is applied as said PVA dichroic film and cover sheets arebrought into contact, thereby forming a polarizer plate web; and

(i) drying the polarizer plate web.

The present process is capable of providing high quality lamination and,at the same time, can be performed continuously at relatively elevatedspeeds, such that a robust and economic manufacture of high qualitypolarizing plates is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary coating and drying apparatus thatcan be used in the practice of the method of the present invention;

FIG. 2 is a schematic of an exemplary coating and drying apparatus as inFIG. 1 but also including a station where an alternate winding operationfurther comprises application of a strippable protection layer;

FIG. 3 is a schematic of an exemplary multi-slot coating apparatus thatcan be used in the practice of the present invention;

FIG. 4 is a schematic of one embodiment of the present processcomprising web peeling, conveyance, and lamination, without employingaccumulators at the carrier web winders;

FIG. 5 is a schematic of another embodiment of the present processcomprising web peeling, conveyance, and lamination, differing from theembodiment of FIG. 4 by further comprising the use of accumulators atthe carrier web winders;

FIG. 6 is a schematic of another embodiment of the present processcomprising web peeling, conveyance, and lamination, differing from theembodiment of FIG. 4 by employing a sequential lamination of theprotective cover sheet to the polarizing film;

FIG. 7 is a schematic of another embodiment of the present processcomprising web peeling, conveyance, and lamination, differing from theembodiment of FIG. 4 by not employing accumulators after the unwindingspindles for the supply rolls, and instead employing an accumulatorafter the lamination station;

FIG. 8 shows a cross-sectional representation of a liquid crystal cellwith polarizer plates, which can be made in accordance with the presentinvention, on either side of the cell;

FIG. 9 is a schematic of one embodiment of a double layer butt splicethat may be used in practicing the present invention;

FIGS. 10 a, 10 b are schematic embodiments of a double-layer overlapsplice that may be used in practicing the present invention;

FIG. 11 is a schematic of an embodiment of a peeling station comprisingfilm peeling with a nip roller that can be used instead of a singleroller shown in previous FIGS. 4 to 7;

FIGS. 12 a, 12 b are schematics of yet another embodiment of a peelingstation comprising a knife edge that can be used instead of the peelingstations shown in previous figures;

FIG. 13 is a schematic of a logic diagram for accomplishing improvedtension control following the peeling point (in cross-section);

FIG. 14 shows a cross-sectional representation of a three-layer coversheet that can be used in the invention;

FIG. 15 shows a cross-sectional representation of a guarded cover sheet,which can be used in the invention, comprising a three-layer cover sheetand a partially peeled carrier substrate;

FIG. 16 shows a cross-sectional representation of a guarded cover sheet,which can be used in the invention, comprising a four-layer cover sheetand a partially peeled carrier substrate; and

FIG. 17 shows a cross-sectional representation of a guarded cover sheet,which can be used in the invention, comprising a four-layer cover sheetand a partially peeled carrier substrate wherein the carrier substratehas a release layer formed thereon; and

DETAILED DESCRIPTION OF THE INVENTION

The following definitions apply to the description herein:

In-plane phase retardation, R_(in), of a layer is a quantity defined by(nx−ny)d, where nx and ny are indices of refraction in the direction ofx and y; x is taken as the direction of maximum index of refraction inthe x-y plane and y direction is taken perpendicular to it; the x-yplane is parallel to the surface plane of the layer; and d is athickness of the layer in the z-direction. The quantity (nx−ny) isreferred to as in-plane birefringence, Δn_(in). The value of Δn_(in) isgiven at a wavelength λ=550 nm.

Out of-plane phase retardation, R_(th), of a layer is a quantity definedby [nz−(nx+ny)/2]d, where nz is the index of refraction in thez-direction. The quantity [nz−(nx+ny)/2] is referred to as out-of-planebirefringence, Δn_(th). If nz>(nx+ny)/2, Δn_(th) is positive (positivebirefringence), and thus the corresponding R_(th) is also positive. Ifnz<(nx+ny)/2, Δn_(th) is negative (negative birefringence) and R_(th) isalso negative. The value of Δn_(th) is given at λ=550 nm.

Intrinsic Birefringence, Δn_(int), of a polymer refers to the quantitydefined by (ne−no), where ne and no are the extraordinary and theordinary index of the polymer, respectively. The actual birefringence(in-plane Δn_(in) or out-of-plane Δn_(th)) of a polymer layer depends onthe process of forming it, thus the parameter Δn_(int).

Amorphous is defined as a lack of long-range order. Thus, an amorphouspolymer does not show long-range order as measured by techniques such asX-ray diffraction.

Transmission is a quantity to measure the optical transmissivity. It isgiven by the percentile ratio of out coming light intensity lout toinput light intensity I_(in) as I_(out)/I_(in)×100.

Optic Axis refers to the direction in which propagating light does notsee birefringence.

Uniaxial is defined as two of the three indices of refraction, nx, ny,and nz, are essentially the same.

Biaxial is defined as that the three indices of refraction, nx, ny, andnz, are all different.

Acid number for a polymer is defined as the number of milligrams of KOHrequired to neutralize 1 gram of polymer solids.

Cover sheets employed in Liquid Crystal Displays are typically polymericsheets having low optical birefringence that are employed on each sideof a dichroic PVA film in order to maintain the dimensional stability ofthe dichroic PVA film and to protect it from moisture and UVdegradation. In the following description a guarded cover sheet isdefined as a cover sheet that is disposed on a removable, protectivecarrier substrate. A strippable, protective film may also be employed onthe side of the cover sheet opposite to the carrier substrate so thatboth sides of the cover sheet are protected prior to its use in apolarizer plate.

A layer promoting adhesion to PVA is a distinct layer that is applied ina coating step either separate from or simultaneous with the applicationof the low birefringence protective polymer film. The layer promotingadhesion to PVA provides acceptable adhesion of the cover sheet to adichroic PVA film (in a liquid crystal display application) without theneed for a wet pretreatment, such as saponification, of the cover sheetprior to lamination to the PVA film.

An optional tie layer is a distinct layer that is applied in a coatingstep either separate from or simultaneous with the application of thelow birefringence protective polymer film or layer promoting adhesion toPVA.

The present invention relates to an improved method of using theprotective cover sheet to fabricate polarizers (polarizers are alsoreferred to as polarizing plates or polarizer plates). The protectivecover sheet used in the invention comprises a low birefringenceprotective polymer film and a layer promoting adhesion to polyvinylalcohol films, preferably comprising a hydrophilic polymer. In otherembodiments, the protective cover sheet can optionally further comprisea tie layer or one or more auxiliary layers. Suitable auxiliary layersfor use in the present invention include an abrasion resistant hardcoatlayer, antiglare layer, anti-smudge layer or stain-resistant layer,antireflection layer, low reflection layer, antistatic layer, viewingangle compensation layer, and moisture barrier layer.

The protective cover sheet used in the present process is provided as aguarded cover sheet composite comprising a carrier substrate and theprotective cover sheet. Optionally, the guarded cover sheet composite ofthe invention also comprises a strippable, protection layer on the sideof the cover sheet opposite to the carrier substrate. The guarded coversheet composite is particularly advantageous and effective when the lowbirefringence protective polymer film is thin, for example, when thethickness is about 40 micrometers or less, preferably about 20 to 30micrometers.

Turning now to FIG. 1 there is shown a schematic of an exemplary andwell-known coating and drying system 10 suitable for preparing the coversheets used in the present invention. The coating and drying system 10may be used to apply very thin films to a moving carrier substrate 12and to subsequently remove solvent in a dryer 14. A single coatingapparatus 16 is shown such that coating and drying system 10 has onlyone coating application point and only one dryer 14, but two or three(even as many as six) additional coating application points withcorresponding drying sections are known in the fabrication of compositethin films. The process of sequential application and drying is known inthe art as a tandem coating operation.

Coating and drying system 10 includes an unwinding station 18 to feedthe moving substrate 12 around a back-up roller 20 where the coating isapplied by coating apparatus 16. The coated substrate 22 then proceedsthrough the dryer 14. In one embodiment of the present invention, theguarded cover sheet composite 24 comprising a cover sheet on substrate12 is wound into rolls at a wind-up station 26.

As depicted, an exemplary four-layer coating is applied to movingsubstrate 12. Coating liquid for each layer is held in respectivecoating supply vessel 28, 30, 32, 34. The coating liquid is delivered bypumps 36, 38, 40, 42 from the coating supply vessels to the coatingapparatus 16 via conduits 44, 46, 48, 50, respectively. In addition,coating and drying system 10 may also include electrical dischargedevices, such as corona or glow discharge device 52, or polar chargeassist device 54, to modify the moving substrate 12 prior to applicationof the coating.

Turning next to FIG. 2 there is shown a schematic of the same exemplarycoating and drying system 10 depicted in FIG. 1 with an alternativewinding operation to apply a strippable protection layer. Accordingly,the figures are numbered identically up to the winding operation. In thepractice of the present invention the guarded cover sheet composite 24comprising a carrier substrate (which may be a resin film, paper,resin-coated paper, or metal) with a cover sheet applied thereto istaken between opposing nip rollers 56, 58. The guarded cover sheetcomposite 24 is adhesively adhered or electrostatically adhered to apreformed strippable protection layer 60 which is supplied fromunwinding station 62 and the guarded cover sheet composite containingthe strippable protection layer is wound into rolls at wind-up station64. Preferably, polyolefin or polyethylene phthalate (PET) is used asthe preformed, strippable protection layer 60. Either the guarded coversheet composite 24 or the preformed strippable protection layer 60 maybe pretreated with an electric charge generator to enhance theelectrostatic attraction of the preformed strippable protection layer 60to the guarded cover sheet composite 24.

The coating apparatus 16 used to deliver coating fluids to the movingsubstrate 12 may be a multi-layer applicator such as a slide beadhopper, as taught for example in U.S. Pat. No. 2,761,791 to Russell, ora slide curtain hopper, as taught by U.S. Pat. No. 3,508,947 to Hughes.Alternatively, the coating apparatus 16 may be a single layerapplicator, such as slot die bead hopper or jet hopper.

As mentioned above (FIGS. 1 and 2), coating and drying system 10includes a dryer 14 that will typically be a drying oven to removesolvent from the coated film. An exemplary dryer 14 includes a firstdrying section 66 followed by eight additional drying sections 68-82capable of independent control of temperature and air flow. Althoughdryer 14 is shown as having nine independent drying sections, dryingovens with fewer compartments are well known and may be used to practicethe method of the present invention. The dryer 14 can have two or moreindependent drying zones or sections.

Preferably, each of drying sections 66-82 each has independenttemperature and airflow controls. In each section, temperature may beadjusted between 5° C. and 150° C. To minimize drying defects from casehardening or skinning-over of the wet layers, optimum drying rates areneeded in the early sections of dryer 14. There are a number ofartifacts created when temperatures in the early drying zones areinappropriate. For example, fogging or blush of cellulose acetate filmsis observed when the temperature in zones 66, 68 and 70 are set at 25°C. This blush defect is particularly problematic when high vaporpressures solvents (methylene chloride and acetone) are used in thecoating fluids. Aggressively high temperatures of 95° C. in the earlydrying sections 66, 68, and 70 tend to cause premature delamination ofthe cover sheet from the carrier substrate. Higher temperatures in theearly drying sections are also associated with other artifacts such ascase hardening, reticulation patterns and blistering of the cover sheet.

In a preferred embodiment, the first drying section 66 is operated at atemperature of at least about 25° C. but less than 95° C. with no directair impingement on the wet coating of the coated web 22. In anotherpreferred embodiment, drying sections 68 and 70 are also operated at atemperature of at least about 25° C. but less than 95° C. It ispreferred that initial drying sections 66, 68 be operated attemperatures between about 30° C. and about 60° C. It is most preferredthat initial drying sections 66, 68 be operated at temperatures betweenabout 30° C. and about 50° C. The actual drying temperature in dryingsections 66, 68 may optimize empirically within these ranges by thoseskilled in the art.

Referring now to FIG. 3, a schematic of an exemplary coating apparatus16 is shown in detail. Coating apparatus 16, schematically shown in sideelevational cross-section, includes a front section 92, a second section94, a third section 96, a fourth section 98, and a back plate 100. Thereis an inlet 102 into second section 94 for supplying coating liquid tofirst metering slot 104 via pump 106 to thereby form a lowermost layer108. There is an inlet 110 into third section 96 for supplying coatingliquid to second metering slot 112 via pump 114 to form layer 116. Thereis an inlet 118 into fourth section 98 for supplying coating liquid tometering slot 120 via pump 122 to form layer 124. There is an inlet 126into back plate 100 for supplying coating liquid to metering slot 128via pump 130 to form layer 132. Each slot 104, 112, 120, 128 includes atransverse distribution cavity. Front section 92 includes an inclinedslide surface 134, and a coating lip 136. There is a second inclinedslide surface 138 at the top of second section 94. There is a thirdinclined slide surface 140 at the top of third section 96. There is afourth inclined slide surface 142 at the top of fourth section 98. Backplate 100 extends above inclined slide surface 142 to form a back landsurface 144. Residing adjacent the coating apparatus or hopper 16 is acoating back-up roller 20 about which a substrate 12 is conveyed.Coating layers 108, 116, 124, 132 form a multi-layer composite sheetthat forms a coating bead 146 between coating lip 136 and substrate 12.Typically, the coating apparatus 16 is movable from a non-coatingposition toward the coating back-up roller 20 and into a coatingposition. Although coating apparatus 16 is shown as having four meteringslots, coating dies having a larger number of metering slots (as many asnine or more) are well known and may be used to practice the method ofthe present invention.

The coating fluids for the low birefringence protective polymer film arecomprised principally of a polymer binder dissolved in an organicsolvent. In a particularly preferred embodiment, the low birefringenceprotective polymer film is a cellulose ester. These are commerciallyavailable in a variety of molecular weight sizes as well as in the typeand degree of alkyl substitution of the hydroxyl groups on the cellulosebackbone. Examples of cellulose esters include those having acetyl,propionyl and butyryl groups. Of particular interest is the family ofcellulose esters with acetyl substitution known as cellulose acetate. Ofthese, the fully acetyl substituted cellulose having a combined aceticacid content of approximately 58.0-62.5% is known as triacetyl cellulose(TAC) and is generally preferred for preparing cover sheets used inelectronic displays.

In terms of organic solvents for TAC, suitable solvents, for example,include chlorinated solvents (methylene chloride and 1,2dichloroethane), alcohols (methanol, ethanol, n-propanol, isopropanol,n-butanol, isobutanol, diacetone alcohol and cyclohexanol), ketones(acetone, methylethyl ketone, methylisobutyl ketone, and cyclohexanone),esters (methyl acetate, ethyl acetate, n-propyl acetate, isopropylacetate, isobutyl acetate, n-butyl acetate, and methylacetoacetate),aromatics (toluene and xylenes) and ethers (1,3-dioxolane,1,2-dioxolane, 1,3-dioxane, 1,4-dioxane, and 1,5-dioxane). In someapplications, small amounts of water may be used. Normally, TACsolutions are prepared with a blend of one or more of the aforementionedsolvents. Preferred primary solvents include methylene chloride,acetone, methyl acetate, and 1,3-dioxolane. Preferred co-solvents foruse with the primary solvents include methanol, ethanol, n-butanol andwater.

Coating formulations may also contain plasticizers. Appropriateplasticizers for TAC films include phthalate esters (dimethylphthalate,dimethoxyethyl phthalate, diethylphthalate, dibutylphthalate,dioctylphthalate, didecylphthalate and butyl octylphthalate), adipateesters (dioctyl adipate), phosphate esters (tricresyl phosphate,biphenylyl diphenyl phosphate, cresyl diphenyl phosphate, octyl diphenylphosphate, tributyl phosphate, and triphenyl phosphate), and glycolicacid esters (triacetin, tributyrin, butyl phthalyl butyl glycolate,ethyl phthalyl ethyl glycolate, and methyl phthalyl ethyl glycolate).Non-aromatic ester plasticizers as described in commonly assignedco-pending U.S. patent application Ser. No. 10/945,305. Plasticizers arenormally used to improve the physical and mechanical properties of thefinal film. In particular, plasticizers are known to improve theflexibility and dimensional stability of cellulose acetate films.However, plasticizers are also used as coating aids in the convertingoperation to minimize premature film solidification at the coatinghopper and to improve drying characteristics of the wet film.Plasticizers are used to minimize blistering, curl and delamination ofTAC films during the drying operation. In a preferred embodiment of thepresent invention, plasticizers are added to the coating fluid at atotal concentration of up to 50% by weight relative to the concentrationof polymer in order to mitigate defects in the final TAC film.

The coating formulation for the low birefringence protective polymer mayalso contain one or more UV absorbing compounds to provide UV filterelement performance and/or act as UV stabilizers for the lowbirefringence protective polymer film. Ultraviolet absorbing compoundsare generally contained in the polymer in an amount of 0.01 to 20 weightparts based on 100 weight parts of the polymer containing no ultravioletabsorber, and preferably contained in an amount of 0.01 to 10 weightparts, especially in an amount of 0.05 to 2 weight parts. Any of thevarious ultraviolet light absorbing compounds which have been describedfor use in various polymeric elements may be employed in the polymericelements of the invention, such as hydroxyphenyl-s-triazine,hydroxyphenylbenzotriazole, formamidine, or benzophenone compounds. Asdescribed in commonly assigned U.S. Pat. No. 6,872,766, the use ofdibenzoylmethane ultraviolet absorbing compounds in combination with asecond UV absorbing compound such as those listed above have been foundto be particularly advantageous with respect to providing both a sharpcut off in absorption between the UV and visible light spectral regionsas well as increased protection across more of the UV spectrum.Additional possible UV absorbers which may be employed includesalicylate compounds such as 4-t-butylphenylsalicylate; and[2,2′-thiobis-(4-t-octylphenolate)]n-butylamine nickel(II). Mostpreferred are combinations of dibenzoylmethane compounds withhydroxyphenyl-s-triazine or hydroxyphenylbenzotriazole compounds.

Coating formulations may also contain surfactants as coating aids tocontrol artifacts related to flow after coating. Artifacts created byflow after coating phenomena include mottle, repellencies, orange-peel(Bernard cells), and edge-withdraw. Surfactants used control flow aftercoating artifacts include siloxane and fluorochemical compounds.Examples of commercially available surfactants of the siloxane typeinclude: (1) Polydimethylsiloxanes such as DC200 FLUID from Dow Corning;(2) Poly(dimethyl, methylphenyl)siloxanes such as DC510 FLUID from DowCorning; (3) Polyalkyl substituted polydimethysiloxanes such as DC190and DC1248 from Dow Corning as well as the L7000 SILWET series (L7000,L7001, L7004 and L7230) from Union Carbide; and (4) Polyalkylsubstituted poly(dimethyl, methylphenyl)siloxanes such as SF1023 fromGeneral Electric. Examples of commercially available fluorochemicalsurfactants include: (1) Fluorinated alkyl esters such as the FLUORADseries (FC430 and FC431) from the 3M Corporation; (2) Fluorinatedpolyoxyethylene ethers such as the ZONYL series (FSN, FSN100, FSO,FSO100) from DuPont; (3) Polyperfluoroalkyl ethylacrylates such as the Fseries (F270 and F600) from NOF Corporation; and (4) Perfluoroalkylderivatives such as the SURFLON series (S383, S393, and S8405) from theAsahi Glass Company. Surfactants are preferably of the non-ionic type,and either the siloxane or fluorinated type can be added to theuppermost layers.

In terms of surfactant distribution, surfactants are most effective whenpresent in the uppermost layers with respect to a multi-layer coating.In the uppermost layer, the concentration of surfactant is preferably0.001-1.000% by weight and most preferably 0.010-0.500%. In addition,lesser amounts of surfactant may be used in the second uppermost layerto minimize diffusion of surfactant into the lowermost layers. Theconcentration of surfactant in the second uppermost layer is preferably0.000-0.200% by weight and most preferably between 0.000-0.100% byweight. Because surfactants are only necessary in the uppermost layers,the overall amount of surfactant remaining in the final dried film issmall.

Although surfactants are not required to practice the method of thecurrent invention, surfactants do improve the uniformity of the coatedfilm. In particular, mottle nonuniformities are reduced by the use ofsurfactants. In transparent cellulose acetate films, mottlenonuniformities are not readily visualized during casual inspection. Tovisualize mottle artifacts, organic dyes may be added to the uppermostlayer to add color to the coated film. For these dyed films,non-uniformities are easy to see and quantify. In this way, effectivesurfactant types and levels may be selected for optimum film uniformity.

The preparation of the cover sheet and guarded cover sheet compositeused in the present invention may also include the step of coating overa previously prepared (by coating or casting process) cover sheet film.For example, the coating and drying system 10 shown in FIGS. 1 and 2 maybe used to apply a second multi-layer film to an existing lowbirefringence protective polymer film or cover sheet composite. If thefilm or cover sheet composite is wound into rolls before applying thesubsequent coating, the process is called a multi-pass coatingoperation. If coating and drying operations are carried out sequentiallyon a machine with multiple coating stations and drying ovens, then theprocess is called a tandem coating operation. In this way, thick lowbirefringence protective polymer films may be prepared at high linespeeds without the problems associated with the removal of large amountsof solvent from a very thick wet film. Alternatively, many differentcover sheet configurations having various combinations of auxiliarylayers applied via a tandem or multi-pass coating operation may beprepared. Moreover, the practice of multi-pass or tandem coating alsohas the advantage of minimizing other artifacts such as streak severity,mottle severity, and overall film nonuniformity.

Turning now to FIG. 4, a schematic representation of one embodiment of amethod according to the present invention for fabricating a polarizerplate from guarded cover sheet composites of the invention isillustrated. FIG. 4 shows a roll-to-roll process involving web peelingat a peeling station and lamination together of a plurality of webs at alamination station. For one side of the polarizer plate, guarded coversheet composite 151 (see FIG. 15) comprising cover sheet 171 and carriersubstrate 170 and, for the opposite side of the polarizer plate, aguarded cover sheet composite 153 (see FIG. 16) comprising cover sheet173 and carrier substrate 170 b are supplied from supply rolls 200 a and200 b, respectively. A PVA-dichroic film 202 is stretched and dyed withiodide to from a dichroic film in a continuous process prior to beingsupplied to the lamination station. Prior to entering a lamination nipbetween opposing pinch rollers 206 and 208, the carrier substrates 170 aand 170 b are peeled, at a peeling station, from guarded cover sheetcomposites 151 and 153 to expose a lowermost layer (in the case of FIGS.15 and 16, this is layer 162, which is the layer promoting adhesion toPVA). The peeled carrier substrates 170 a, 170 b are wound onto rolls onthe carrier winders 210 a, 210 b. A glue solution may be optionallyapplied to both sides of the PVA-dichroic film or to the lowermost layerof cover sheets 171 and 173 prior to the sheets and film entering thenip between pinch rollers 206 and 208. The glue solution may be appliedby a variety of well known coating methods, including but not limited tospray coating, drip coating, hopper coating, roller coating, bladecoating, wire rod coating, etc.

Cover sheets 171 and 173 are laminated to either side of dichroic PVAfilm 202 with the application of pressure (and, optionally, heat)between the opposing pinch rollers 206 and 208 to give the polarizerplate sheet 250 (which sheets can eventually be finished into individualpolarizer plates, such as rear polarizer plate 252 and front polarizerplate 254 in FIG. 18, for individual display devices). Polarizer platesheet 250 via conveyance roller 211 may then be conveyed to dryers (notshown) for drying, preferably by heating, and wound into rolls untilneeded. One possible embodiment of the dryers is described in copendingU.S. Ser. No. 11/066,785, hereby incorporated by reference. Depending onthe particular layer configuration for the guarded cover sheetcomposites employed, a wide variety of polarizer plates having coversheets with various combinations of auxiliary layers may be fabricated.

In this particular embodiment, the carrier substrate is preferably apolyester and the protective cover sheet comprises a protective polymersuch as TAC plus optional auxiliary layers. The protective cover sheetcomprises at least a low birefringence polymer protective film and alayer promoting adhesion to polyvinyl alcohol films.

In the embodiment of FIG. 4, the peeling station comprises peelingrollers 212 a, 212 b. FIG. 4 further shows a web spreading stationcomprising bowed (bending) roller 214 a, 214 b, before the laminationnip, to reduce draw-lines and creases. Following peeling, the carrierweb that has been wound onto the carrier winders 210 a, 210 b can bediscarded or recycled at a later time.

In FIG. 4, each of the composite sheets (guarded cover sheet composites)151, 153 is conveyed in proximity to a means for double-sided splicing216 a, 216 b, wherein said means functions to double splice eachcomposite sheet, when expiring, with fresh web. When each of said firstor second composite sheet 151, 153 is near to expiring, which can occurindependently, each composite sheet is secured to said means for doublesplicing and then each such expiring composite sheet is double spliced,preferably butt sliced, with a fresh composite sheet such that peelingand laminating steps to follow are maintained in continuous operation.

The means for double splicing can be isolated between supply roll 200 a,200 b and a means for isolating tension during splicing, for example, aclamp 219 a, 219 b. After the means for isolating tension duringsplicing, each of the composite sheets is conveyed through anaccumulator 218 a, 218 b positioned between clamp 219 a, 219 b and driveroller 220 a, 220 b.

Following the double-sided splicing means 216 a, 216 b and clamp 219 a,219 b, the accumulator 218 a, 218 b supplies in uninterrupted fashion alaminating station comprising the pinch rollers 206, 208 during thesplice cycle. After the accumulator 218 a and 218 b, the compositesheets 151, 153 are then conveyed into a drive roller 220 a, 220 b thatis used to control the lamination tension of the cover sheet 171 and173, respectively. The first feedback signal 276 a, 276 b is used forcontrolling drive speed of supply rolls 200 a and 200 b. The secondfeedback signal 278 a, 278 b, from load cell roller 215 a, 215 b, forcontrolling the drive speed of drive roller 220 a, 220 b is described inmore detail below with respect to FIG. 13.

In the embodiment of FIG. 4, prior to the splice sequence, the expiringroll starts its deceleration and the accumulator starts to closeallowing the rest of the machine to remain at line speed. The expiringweb at this point reaches zero speed. Optionally, sensors andcontrollers can be employed to bring the fresh web into alignment withthe expiring web. See, for example, U.S. Pat. No. 6,016,989 to Gangemi,hereby incorporated by reference. The expiring web is then connected tothe fresh web by double layer splicing, as illustrated in FIG. 9, eithermanually or by an automatic equipment. Alternatively, a double layer lapsplice can be applied wherein the fresh web is hand peeled for a shortlength and the expiring web is sandwiched and adhered between apre-peeled fresh web as described in more detail below with respect toFIGS. 10 a and 10 b. The fresh web can be provided from fresh supplyroll 201 a, 201 b of the spindle 203 a, 203 b, respectively.

After the splice operation, the start of web conveyance involves thefresh web supply roll 201 a, 201 b accelerating up to a speed greaterthan line speed allowing the accumulator 218 a, 218 b to fill with freshweb.

To get ready for the next splice operation, the fresh supply roll 201 a,201 b rotates into position. The operator replaces an expiring roll witha fresh roll, allowing the splice cycle to be repeated again. Thisallows a continuous operating of the peeling and laminating operationdownstream of the splicing operation.

With respect to the double splicing operation, preferably this entails abutt splice such as shown in FIG. 9 or an overlap splice as shown inFIGS. 10 a and 10 b. Overlap (lap) and butt splices are two common typesof splices for webs of sheet materials which splices can be madeautomatically or manually or a combination of partially automatic andpartially manual. For an overlap splice, the trailing and leading edgesof, respectively, the expiring and fresh rolls of sheet material aretypically joined together, for example, by a one-sided adhesive materialor by a double-sided adhesive tape. The double-sided adhesive tape canbe either a double-coated adhesive tape having a backing member and anadhesive layer on each side or a transfer tape with a single adhesivelayer. Such tapes are typically supplied with a single release liner.

Referring more particularly to FIG. 9, one embodiment of a typicalpermanent butt splice is shown in the figure. The butt splicing wouldinvolve the following steps (either manually or by an automated device).A cut is made in the fresh web 222 (comprising fresh carrier sheet 224,fresh adhesive layer 226, and fresh cover sheet 228) to expose astraight edge across the width (perpendicular to machine runningdirection). Once the expiring roll stops rotating (start of the splicecycle), a straight cut is made on the expiring web 232 (comprisingexpiring cover sheet 244, expiring carrier sheet 246, and expiringadhesive layer 247) to expose a straight edge across the width. Theexposed edges of the expiring web and fresh web are aligned.

Alternatively, these steps can be done (once the expiring roll stopsrotating at the start of the splice cycle) by pulling expiring web 232and overlaying it with the fresh web 222, then making a straight cutthrough the expiring web 232 and the fresh web 222 to expose a straightedge across the width on both webs. After making the cut, the tails areremoved.

A first single-sided tape 238 a is used to connect the two cover sheetsacross the entire width. A second single-sided tape 238 b is used toconnect the two carrier substrates all across the width. Thesingle-sided tape 238 a 238 b comprises glue-containing layer 242 a, 242b, and backing support 240 a, 240 b. The splicing cycle is nowcompleted, and the double layer splice can be peeled throughcontinuously at the peeling station. Various other types of buttsplicing tapes are disclosed in U.S. Pat. No. 5,212,002 and in the priorart, or known to the skilled artisan.

Referring now to FIGS. 10 a and 10 b, a typical lap splicing operationmay involve the following process steps (either manually or by anautomated device). A cut is made in the fresh web 222 (comprising freshcarrier sheet 224, fresh adhesive layer 226 and fresh cover sheet 228)to expose a straight edge across the width (perpendicular to the machinerunning direction). A short length of the fresh web is peeled toseparate the fresh cover sheet 228 from the carrier web 224, therebyforming a pre-peeled cover sheet portion 234 and pre-peeled carrier webportion 236. Once the expiring roll stops rotating (start of the splicecycle), a straight cut is made on the expiring web 232 to expose astraight edge across the width.

When using the single-sided tape 238 a, 238 b (FIG. 10 a), the followingsteps are performed: The expiring web 232 (no pre-peeling necessary) ispositioned between the pre-peeled cover sheet portion 234 and pre-peeledcarrier web portion 236 from the fresh roll, making sure the webs arewell-aligned.

The single-sided tape 238 a, 238 b, comprising tape support layer 240 a,240 b and glue-containing layer 242 a, 242 b is employed to connect two(fresh and expiring) web composites 222 and 232, each comprising aguarded cover sheet composite 151 or 153, all across the width.

Referring to FIG. 10 b, when using a double-sided tape, the followingsteps can be performed. Two double-sided tapes 274 a, 274 b are placedon the expiring web 232 across the web width, one on top of the expiringcover sheet 244 and the other on top of the expiring carrier sheet 246.The expiring web 232 (with tapes) are positioned between the pre-peeledcover sheet portion 234 and the pre-peeled carrier web portion 236 fromthe fresh roll, making sure the webs align well. The pre-peeled freshcover sheet portion 234 of the fresh roll is adhered to the expiringcover sheet 244 on the expiring web 232 by the application of pressure,and the pre-peeled carrier web portion of the fresh roll is adhered tothe carrier web 246 on the expiring web also by pressure. The splicingcycle is then complete, and the double layer splice can be peeledthrough continuously at the peeling station.

Such web splicing apparatus accomplishes double splicing of the expiringweb with fresh web wherein the line speed on a downstream output side ofthe accumulator does not substantially fluctuate from a predeterminedlamination velocity through the other side of drive rollers 220 a, 220 bin FIG. 4. In order to achieve this object, the splicing method caninclude storing a predetermined length of the first and second web in anaccumulator.

One common type of accumulator, as exemplified in FIG. 4, comprisesupper and lower accumulator rollers 217 a, 217 b, at least one of whichcan be moveable, and upper and lower mounting frames 221 a, 221 b. Achange in an amount of the web stored in the accumulator can bedetermined from a positional change of the movable roller. Specifically,when one or more moveable rollers are moving away from one or more fixedrollers associated with the accumulator, the velocity at which the webis fed out from the supply roll is greater than the velocity at whichthe web is output from the accumulator (e.g., a line velocity), therebyincreasing the amount of the web stored in the accumulator. On the otherhand, when the movable rollers are moving toward the fixed rollers, thevelocity at which the web is fed out from the supply roll is less thanthe line velocity, thereby decreasing the amount of the web stored inthe accumulator. Thus, when the velocity at which the web is fed outfrom the supply roll is greater than the line velocity, the amount ofthe web stored in the accumulator increases, whereas when the velocityat which the web is fed out from the supply roll is smaller than theline velocity, the amount of the web stored in the accumulatordecreases.

Accordingly, there is a predetermined relationship between thepositional change of the movable rollers (the change per unit time: thevelocity of the movable rollers) and the velocity at which the web isfed out from the supply roll. In one embodiment, the velocity at whichthe web is fed out from the supply roll is equal to the diameter of theroll multiplied by the angular velocity of the roll, whereby thediameter of the roll can be known from the positional change of themovable rollers, the line velocity, and the angular velocity.

Referring still to FIG. 4, the accumulator 218 a, 218 b can be ascissor-type comprising accumulator rollers 217 a, 217 b that aremovable and a frame 221 a, 221 b, to which the movable rollers 217 a,217 b are attached. Alternatively, the accumulator can include aplurality of movable rollers and a plurality of fixed rollers, aroundwhich the web is passed, and a frame to which the movable rollers areattached, as disclosed for example, in US 2003/0209629 A1, herebyincorporated by reference. The composite sheets 151, 153 in FIG. 4 maybe placed under tension by a self-weight of the bottom of frame 221 a,221 b, or alternatively, an elastic member (not shown) such as a springor a damper, or a weight, may be attached to the frame 221 a, 221 b forapplying a predetermined tension on the composite sheets 151, 153.

In the accumulator 218 a and 218 b, the composite sheet 151, 153 ispassed around the rollers in a zigzag pattern, and the composite sheet151, 153 is placed under a predetermined tension as the opposing rollers217 a, 217 b connected together via the frames 221 a, 221 b are movedcloser or further away. For example, when more composite sheet 151, 153is supplied to the accumulator 218 a, 218 b than is output, the rollers217 a, 217 b are moved apart. On the other hand, when more compositesheet 151, 153 is output from the accumulator than is supplied, therollers are moved toward each other. In other words, the accumulator 218a, 218 b can store a predetermined or controllable length of thecomposite sheet 151, 153, and the composite sheet 151, 153 can be outputfrom the accumulator even if the flow velocity of the composite sheet151, 153 is zero at the position of input of accumulator 218 a, 218 b.As a result, the tension on the composite sheet 151, 153 can be kept ata predetermined value.

Moreover, as the number of the rollers 217 a, 217 b is larger, morecomposite sheet 151, 153 can be stored in the accumulator 218 a, 218 b.However, as the number of the rollers 217 a, 217 b is larger, thetension that can be applied onto the composite sheet 151, 153 by theload on the rollers 217 a, 217 b are smaller. Thus, as the number of therollers 217 a, 217 b are increased, it is necessary to increase the loadapplied onto the rollers which are connected together.

The accumulator 218 a, 218 b may be of a type illustrated in FIG. 4, inwhich the movable rollers are moved up and down. Alternatively, theaccumulator 218 a, 218 b may be of a type in which a supporting rodholds movable rollers, and a pivoting section allows for a pivotalmovement of the supporting rod, as disclosed for example, in US2003/0209629 A1. The web may be placed under tension by the self-weightof the supporting rod. Alternatively, an elastic member such as a springor a damper, or a weight, may be attached to the supporting rod forapplying a predetermined tension on the web, wherein the elastic memberis attached at or near one end of the supporting rod that is oppositefrom the pivoting section.

Normally, the accumulator 218 a, 218 b in FIG. 4 stores an averageamount of the composite sheet 151, 153 between the maximum amount andthe minimum amount of the composite sheet 151, 153 that can be storedtherein, so that fluctuations in the amount of the composite sheet 151,153 supplied to the peeling and laminating stations can be optimallyaccommodated. Before the expiring and fresh webs are spliced together, acontroller (not shown) increases the rotational speed of the firstdriver for the expiring roll so that a predetermined amount of thecomposite sheet 151, 153 that is greater than the above-mentionedaverage amount is stored in the accumulator 218 a, 218 b. Thepredetermined amount may be of any value as long as it provides asufficient amount of extra time for splicing the expiring web and thefresh web together.

A controller can be used to turn off the driver when a predeterminedamount of the expiring web is stored in the accumulator 218 a, 218 b inpreparation for the splicing of the fresh and expiring webs. When theexpiring roll stops spinning, the controller can control the splicer 216a, 216 b so that it splices the fresh web to the expiring web and cutsoff the expiring web. The amount of the expiring web stored in theaccumulator 218 a, 218 b decreases during the splicing operation. Afterthe expiring web is cut off, the controller turns on the second driverfor the fresh roll. Thus, the state of the accumulator 218 a, 218 bchanges during the web splicing process.

After each web is conveyed through the drive rollers 220 a, 220 b, saidcarrier substrate 170 a, 170 b is removed from, respectively, saidprotective cover sheet 171, 173 at a peeling station to produce,respectively, (i) a first and second unguarded web 171 and 173, eachcomprising the protective cover sheet, and (ii) a first and secondcarrier web 170 a, 170 b each comprising the carrier substrate.

The peeling station comprises a peeling means for separating theunguarded web from the carrier web. Such a peeling means can comprise asingle roller 212 a, 212 b as in FIG. 4 or, alternatively, a contactingnip comprised of two rollers as shown in FIG. 11, a knife as shown inFIGS. 12 a and 12 b, or a peeling rod, or other means known in the artfor peeling.

Such peeling methods can be designed to avoid chatterlines, stickyspots, moving separation lines, and other peeling-related defects. InFIG. 11 web peeling occurs from a nip between two rollers 280 and 281.The rollers can be either bare metallic or rubber covered. The coverrubber hardness can vary from 20 to 85 Shore A Durameter. The nippressure can range from 1 Newton per meter web width up to 2000 Newtonsper meter web width. The use of a roller nip to peel webs apart can fixthe position of a separating line (the line where the webs separate),and largely reduces defects due to movement of the separation line.

FIG. 4 shows the peeling of the webs from a single roller 212 a, 212 b.The roller size can impact peeling quality. Peeling off of a largediameter roller often times generates chatterlines and meanderingline-defects at the surface of the webs due to stick and slip behaviorbetween two webs. The generation of chatter-lines can be avoided byreducing the diameter of the peeling roller. The critical diameter atwhich the chatterline may disappear can depend on the adhesive formulabetween the layers, and it can vary typically from 10 mm to 200 mm. Thepeeling roller can be idle or driven. When peeling off a small-sizediameter roller, the roller deflection due to the web tension maygenerate drawlines in the free spans of the web. In that case, a backingroller can be installed to reduce the roller deflection and drawlines.

FIG. 4 shows that, at the peeling station, the carrier web touches thepeeling roller. Alternatively at the peeling station, the cover sheetcan touch the peeling roller. The diameter of the peeling roller and thearrangement of either carrier web or cover sheet touching peeling rollercan be optimized to avoid peeling-related defects such as sticky spots.

FIGS. 12 a and 12 b show the peeling of the webs from a knife-edge 258of stationary knife 256. This is in some sense the extreme of peeling ona stationary roller that is very small in size. The knife-edge 258 canbe used when peeling defects become an issue with even very small sizepeeling rollers. On the other hand, due to the fact that the knife edgecan be fairly wide, the deflection and thus draw-lines would tend to beless of an issue than with small size peeling rollers. The angle (a) ofthe knife surface in the vicinity of the peeling location can range from5 degrees to 170 degrees, and the radius (R) of curvature at the peelingpoint can suitably range from 0.1 mm to 5 mm.

In preferred peeling configurations (FIGS. 4, 11, 12, etc.), the webtensions in each span can typically vary from 1 to 2000 Newtons permeter web width, preferably from 100 to 1000 Newtons per meter webwidth. Web wrap angles typically can vary from 2 to 180 degrees.

Experiments have shown that a large amount of electrostatic charge canbe generated at the peeling location and at the location where a web isbeing unwound. To remove or reduce electrostatic charge generation,various means known to the skilled artisan, for example, an tinsel,alpha string, or ionizer may be placed at various locations. Onepreferred location is following the peeling station or following a bowedroller.

Thus, following the peeling operation, after the peeling point, suitablemeans (not shown in FIG. 4) for removing an electrostatic charge fromthe unguarded web can be employed. Likewise, such a means for removingelectrostatic charge can be provided in proximity to a supply roll, whenthe guarded web is removed from the supply roll. A preferredanti-electrostatic means is an alpha string.

Following the accumulator 218 a, 218 b in FIG. 4, the carrier substrate170 a, 170 b is separated or peeled from cover sheet 171, 173. Theremoved carrier substrate 170 a, 170 b can be wound onto carrier webwinders 210 a, 210 b and subsequently empty core 213 a, 213 b of thespindle 223 a, 223 b.

In one embodiment of the invention, the guarded cover sheet composite isconveyed through an accumulator that is positioned between a supply rollfor the guarded cover sheet composite and a peeling station, preferablybetween a tension isolating means, located after a double-sided splicingmeans, and a drive roller. Optionally, the carrier substrate after beingpeeled is conveyed through a different accumulator positioned betweenthe peeling station and a carrier web winder.

In another embodiment of the invention, the composite polarizer sheet isconveyed through an accumulator positioned between a laminator nip and awinder for the composite polarizer sheet. Preferably, this accumulatoris positioned between the laminator nip and a dryer for the compositepolarizer sheet.

Depending on the position of the one or more accumulators, the peelingstation and the lamination station can also be continuous during steadystate operation.

Thus, accumulators may be used in various locations as illustrated inFIGS. 4 to 7. In comparison to the embodiment of FIG. 4, for example, inthe embodiment shown in FIG. 7, accumulators are not used between thedouble sided splicing and peeling operation. Also, in the embodimentsshown in FIGS. 4 and 5, the removed carrier web can be conveyed with orwithout employing an accumulator and wound onto a winding spindle. Stillalternatively, the removed carrier web can merely be disposed of bybeing sent to a waste chopper, preferably for recycling.

Subsequent to the peeling station in FIG. 4, each unguarded web isconveyed over means for controlling tension, for example a load cellroller or float roller 215 a, 215 b, having feedback control to saidsecond means for isolating tension (tension isolation drive roller 220a, b in FIG. 4).

Following the load cell roller 215 a, b, each unguarded web is thenconveyed over a means for spreading the web, suitably removing wrinkles,which is especially advantageous for relatively thin webs. This may beaccomplished by a variety of means, including a bowed or bending roller,concave roller, flex roller, or the like.

In the embodiment of the invention shown in FIG. 4, following thepeeling station, the unguarded web is conveyed over a means forspreading the web such as a bowed (bending) roller 214 a, 214 b. Thisavoids problems such as wrinkles that are a very common problem withresin film. Because webs have very little lateral compressive stiffness,wrinkles often occur. Web-spreading or anti-wrinkle rollers are calledupon to perform a spectrum of activity from stretching a web, tospreading a web, to allowing a web to flatten out or simply not inducingwrinkles in the first place.

Web spreading devices include, for example, bowed rollers, expandingsurface rollers, rubber spreader rollers, grooved metal rollers, andreverse taper rollers. Bowed rollers are a particularly effective webspreading device for use in the present invention. A bowed roller is aflexible roller of consistent diameter that is mounted on a centralshaft by way of a series of bearings or sleeves along the shaft. When acurvature is induced into the shaft, the result is a bow in the rollerface. Bowed rollers remove wrinkles or spread the web because the webwill attempt to orient itself at a right angle along the face of theroller, thus pulling each side of the web in different directions. Also,as the web wraps the roller, it has to travel a longer distance at thecenter and is therefore tighter.

Each unguarded web 171, 173 in FIG. 4 is then brought, eithersimultaneously or sequentially, into contact with a polarizing web 202comprising a dichroic PVA film such that each layer promoting adhesionto polyvinyl alcohol, in each of said two unguarded webs, are contactedwith said polarizing web, for example, a dichroic PVA film, whereinpressure is applied as said PVA dichroic film and cover sheets arebrought into contact, thereby forming a polarizer plate web 250. In theembodiment of FIG. 4, this occurs simultaneously, whereas in theembodiment of FIG. 6, this is accomplished sequentially.

Following the lamination, the polarizer plate web can be dried, eitherby positive measures, usually to speed the process, or by allowing theweb to dry. The polarizing plate web after drying can be wound on awinding spindle (not shown).

Optionally, glue can be applied to the unguarded web or polarizing webprior to contact between the two webs. Alternatively, the glue can beapplied to the unguarded web between the peeling station and thelamination nip. An example of a glue composition is described below.

Optionally, each of said first and second webs may be heated prior topeeling at a peeling point, to facilitate the peeling operation.

The protective cover sheet used in FIGS. 4 to 7 can comprise one or morefunctional/auxiliary layers as described below, for example, a moisturebarrier layer, antistatic layer, compensation layer, hard coat,antiglare, or anti-reflection layer. Usually, the hard coat, antiglare,or anti-reflection layers are on the side of the low birefringencepolymer protective film opposite to the layer promoting adhesion topolyvinyl alcohol films (PVA and crosslinker).

Turning next to FIGS. 5 through 7, there are presented cross-sectionalillustrations showing various cover sheet and guarded cover sheetprocess configurations possible with the present invention. The sameparts in different figures are numbered the same, particularly in FIGS.4 through 7. Hence, for a description of a part not discussed in asubsequent figure, reference is made to previous figures.

The process of FIG. 5 differs from that of FIG. 4 only in that carrierweb 170 a, 170 b, after being peeled from the guarded cover sheetcomposite 171 and 173, is conveyed through a carrier-web accumulatorroller 280 a, 280 b and means for isolating tension, such assupplemental clamp 282 a, 282 b, during roll transfer. A feedback signal286 a, 286 b to carrier winder can be used to control the drive speed ofthe winder 210 a, 210 b, which in turn controls the tension of thecarrier web leading to the winder 210 a, 210 b. An advantage of thisembodiment is that it facilitates the change of rolls, when a windingroll becomes full with carrier web, by allowing zero speed transfer.

Referring next to FIG. 6, each unguarded web 171, 173 is broughtsequentially, rather than simultaneously, into contact with a polarizingweb 202 comprising the dichroic PVA film such that each layer promotingadhesion to polyvinyl alcohol, in each of said two unguarded webs, arecontacted sequentially with the polarizing web, for example, a dichroicPVA film. Pressure is applied as the PVA dichroic film and each coversheet is brought into contact, thereby forming a polarizer plate web250. More particularly, as shown in FIG. 6, unguarded web 171, 173 isfirst laminated to polarizing web 202 between lamination pinch rollers206 and 208. Subsequently, unguarded web 171, 173 is then laminated tothe other side of the polarizing web 202 between downstream laminationpinch rollers 205, 207. As stated above, following the lamination, thepolarizer plate web can be dried, either by positive measures, usuallyto speed the process, or by allowing the web to dry. The polarizingplate web after drying is wound on a winder (not shown).

The other parts in FIG. 6 are as described in previous FIG. 4. Anadvantage of sequential lamination is that nip pressures can beindependently controlled for different cover sheets having differentmaterials, structures, or dimensions.

Referring now to FIG. 7, another design in accordance with the inventionallows a reduction in the number of accumulators, which provides asimpler web path. A post-lamination accumulator 284 is positionedbetween a laminating station represented by lamination pinch rollers206, 208 and a conveyance roller, preferably a downstream bowed roller209. The presence of the post-lamination accumulator 284 allows rolltransfer of the supply rolls 200 a, 200 b and carrier winders 210 a, 210b. However, in this case, the lamination stops temporarily while thewindup of the polarizer plate sheet 250 can occur uninterrupted. Whilelamination stops, the roll transfer can occur at any of the fourspindles 203 a, 203 b and 223 a, 223 b, either manually orautomatically. In this embodiment, control of the lamination station tochange the glue supply or to vary the pressure or timing of laminationmay be desirable to compensate for the temporary line interruption.

The polarizer plate sheet can be wound-up in the form of a web for lateruse. The polarizer plate sheet can be divided into individual platessuch as illustrated by polarizer plates 252 and 254 in FIG. 8, discussedbelow with respect to a typical liquid crystal cell.

As mentioned above, it is desirable to control the tension of at leastthe cover sheet following peeling of the carrier substrate therefrom.Controlling the tension of the cover sheet can comprise sensing aparameter associated with the cover sheet after peeling, which parameterbears a direct or indirect relation (not necessarily explicit) to thetension of the cover sheet, and comparing the sensed parameter to a setpoint. Typically, the parameter is the tension of the cover sheet or isa dimensional or positional characteristic of the cover sheet thatintrinsically is related to tension of the cover sheet. The comparisoncan then be used to generate a signal for controlling a means forconveying the guarded cover sheet composite prior to the peeling. At thesame time, in a preferred embodiment, the speed of the nip rollers inthe lamination station can be maintained by a master drive.

In one embodiment, the conveying means is a drive roller located beforea peeling station for peeling the carrier substrate from the cover sheetand after the supply roll for the guarded cover sheet composite. Inanother embodiment, the conveying means is an unwinder for the guardedcover sheet composite that supplies the guarded cover sheet composite toa peeling station for peeling the carrier substrate from the coversheet. For example, the speed of the conveying means can be increasedwhen the sensed parameter indicates that either the tension of the coversheet is too high or the amount or portion, such as length, of theguarded cover sheet composite in the vicinity of a means for sensing theparameter needs to be increased. Alternatively, the sensed parameter canbe a sensing means that measures a distance related to a displacement ofthe cover sheet in a vicinity of the sensing means. Thus, the sensingmeans can, for example, employ a sensing means that measures the tensionof the cover sheet, either indirectly or directly. Such a sensing meanscan, for example, be a float roller or load cell.

Preferably, the tension of the cover sheet is maintained within a setrange during steady state operation of the method, and the set point isbetween 30 Newtons per meter and 1000 Newtons per meter, preferably 200to 300 Newtons per meter. The variation of the tension of the coversheet that is sensed during steady state operation (that is, after startup and before shut down operation) is preferably maintained within 15percent, more preferably 5 percent, of the set point.

Referring next to FIG. 13, one embodiment of a process control scheme isillustrated for controlling the tension in the cover sheet 173 afterbeing peeled from guarded cover sheet composite 153 at peeling roller212 b. (Thus, the process control for the bottom portion of, forexample, a process according to FIGS. 4 to 7 is being illustrated,whereas the same or similar process control can be used for the topportion of a process such as in FIGS. 4 to 7). In particular, thetension of a cover sheet 173 is controlled by a drive roller 220 b whichcan be viewed as a tension trimmed drive means. A load cell roller 215 bthat functions as a feedback sensor generates a signal proportional tothe amount of web between the laminating pinch rollers (206, 208) andthe drive roller 220 b.

The pinch rollers 206, 208 comprise a master drive 297 that useslamination-nip-drive control block (290) to control its speed relativeto line speed reference 292.

The speed of the drive roller 220 b is varied from line speed reference292 by tension-trimmed-drive control block 294 which maintains thedesired web tension in the cover sheet 173 prior to the lamination pinchrollers 206, 208.

Although the embodiment of FIG. 13 shows a load cell roller 215 b as thefeedback mechanism for controlling the tension trimmed drive, analternative means is a float roller. Still other methods can be used, aswill be appreciated by the skilled artisan. With a load cell, thefeedback signal is typically proportional to web tension. With a floatroller, the feedback signal is typically proportional to a float armposition. In this case, the float arm sets web tension, and the tensiondrive prevents the float roller from running out of travel (slack ortight).

In operation of the process control scheme of FIG. 13, if tension ofunguarded cover sheet 173 is too high, the tension trimmed drive isspeeded up. If tension of 173 is too low, then the tension trimmed drive220 b is slowed down. If the speed of master drive 297 is too highcompared to line speed reference 292, the master drive is slowed down.

As mentioned above, tension of a second cover sheet 171 can becontrolled prior to the lamination nip in a similar fashion.

The control blocks 290 and 294 for the embodiment shown in FIG. 13 areby themselves of conventional design, as will be understood by theskilled artisan. In this embodiment, such a control block comprisesintegrator 305 c comparing a feedback signal 276 b from the load cell215 b to a reference tension 298 (tension set point), the difference ofwhich generates tension error 300. The tension error is converted to aspeed trim 304 via a tension loop software 302, which speed trim isbasically a speed correction that feeds back to an inner speed loopcontrol block comprising an integrator 305 a, speed loop software 306 a,and current loop 308 a that provides electric current to motor 310 awhich drives via gear box 312 a a tension trimmed drive 220 b. The speedtrim 304 together with the line reference speed 292 and the feedbacksignal 314 a from the motor generates the speed error 307 a fed to speedloop software 306.

With respect to control block 290 in FIG. 13, the present embodimentcomprises a drive integrator 305 b that takes the feedback signal 314 bfrom motor 310 b and line speed reference 292 to generate a speed error307 b that feeds to speed loop software 306 b, current loop 308 b, andmotor 310 b, analogously to the control block 294. The motor 310 b, viagear box 312 b controls the master drive 297.

It should be mentioned that in the case of the process of FIG. 7, themaster drive 297 will be a drive roller (not shown) downstream of theaccumulator 284, instead of the lamination pinch rollers 206, 208.

The process of the present invention, as indicated above, can use twocover sheet composites, one on each side of the polarizer film. Thesetwo cover sheet composites can have different structures or materials.The process with respect to each cover sheet, although the same in FIGS.4 to 8, can differ. For example, the process steps with respect to thefirst cover sheet composite 151 shown in FIG. 4, which steps are abovethe polarizer film and up to but not including the lamination station,can be changed to the analogous process steps in FIG. 5 while keepingthe process steps with respect to the second cover sheet composite 153the same. Similarly, the process steps with respect to the first coversheet composite 151 shown in FIG. 6 (shown above the polarizer film inFIG. 6), up to but not including the lamination station, can be changedto the analogous process steps in FIG. 5 while keeping the process stepswith respect to the second cover sheet composite 153 the same.Similarly, other combinations of process steps with respect to the twocover sheet composites, with appropriate modification if necessary, canbe devised as desired under particular circumstances.

Still alternatively, only one half of the process of FIGS. 4 to 7 may beemployed, in which only one cover sheet composites is peeled andlaminated to the polarizer film. In this case, only one cover sheetcomposite enters the pinch rollers 206, 208 in FIG. 4, FIG. 5, and FIG.7. Similarly, the pinch rollers 205, 207 can be eliminated in FIG. 6.The resulting product can be later laminated to other optical films,before or after being wound up.

Turning next to FIGS. 14 through 17, they are presented cross-sectionalillustrations showing various cover sheet and guarded cover sheetconfigurations possible for use with the present invention. FIG. 14shows a cover sheet 189 having lower-most layer 186, intermediate layer188, and uppermost layer 190. In this illustration, layer 186 could be alayer promoting adhesion to PVA, 188 could be a tie layer, and layer 190could be a low birefringence protective polymer film. An optionaladditional layer (not shown), between layer 188 and 190, could be anauxiliary layer such as a viewing angle compensation layer, moisturebarrier layer, abrasion resistant layer, or other type of auxiliarylayer, for example. The cover sheet may be prepared by conventionalcasting methods or by coating methods employing a carrier substrate asdescribed hereinabove.

In FIG. 15, a guarded cover sheet composite 151 comprising a three-layercover sheet 171 having lower-most layer 162, intermediate layer 164, anduppermost layer 168 is shown partially peeled from a carrier substrate170. In this illustration, layer 162 could be a layer promoting adhesionto PVA, layer 164 could be a tie layer, and layer 168 could be a lowbirefringence protective polymer film. Layers 162, 164, and 168 may beformed either by applying and drying three separate liquid layers on thecarrier substrate 170 or by simultaneously applying two or all three ofthe layers and then drying those simultaneously applied layers in asingle drying operation.

In a preferred embodiment, the layer promoting adhesion to PVA is coatedand dried separately from the tie layer and low birefringence protectivepolymer film using a water-based coating formulation. When a cover sheet171 is prepared by coating onto a carrier substrate 170 as illustratedin FIG. 15, it is generally preferred that the layer promoting adhesionto PVA is coated onto the carrier substrate 170 and then dried, prior toapplication of the low birefringence protective polymer film. Auxiliarylayers may be applied either simultaneously with the low birefringenceprotective polymer film or in a subsequent coating and drying operation.

FIG. 16 illustrates another guarded cover sheet composite 153 comprisinga cover sheet 173 that is comprised of, for example, fourcompositionally discrete layers including a lowermost layer 162 nearestto the carrier support 170, two intermediate layers 164 and 166, and anuppermost layer 168. FIG. 16 also shows that the entire multiple layercover sheet 173 may be peeled from the carrier substrate 170. In thisillustration, layer 162 could be a layer promoting adhesion to PVA,layer 164 could be a tie layer, layer 166 could be a low birefringenceprotective polymer film, and layer 168 could be an auxiliary layer suchas an abrasion resistant layer, for example.

FIG. 17 illustrates a further guarded cover sheet composite 159comprising a cover sheet 179 that is comprised of, for example, fourcompositionally discrete layers including a lowermost layer 174 nearestto the carrier substrate 182, two intermediate layers 176 and 178, andan uppermost layer 180. The carrier substrate 182 has been treated witha release layer 184 to modify the adhesion between the cover sheetlowermost layer 174 and substrate 182. Release layer 184 may becomprised of a number of polymeric materials such as polyvinylbutyrals,cellulosics, polyacrylates, polycarbonates andpoly(acrylonitrile-co-vinylidene chloride-co-acrylic acid). The choiceof materials used in the release layer may be optimized empirically bythose skilled in the art.

FIGS. 14 through 17 serve to illustrate some of the guarded cover sheetcomposites that may be constructed based on the detailed teachingsprovided hereinabove. They are not, however, intended to be exhaustiveof all possible variations of the invention. One skilled in the artcould conceive of many other layer combinations that would be useful asguarded cover sheet composites for use in the preparation of polarizerplates for LCDs which various guarded cover sheet composites areamendable to use as a supply web for the process of the presentinvention.

As mentioned above, with respect to the present process, when the coversheet is laminated to the dichroic PVA film such that the layerpromoting adhesion to PVA is on the side of the cover sheet thatcontacts the PVA dichroic film, a glue solution can be used forlaminating the cover film and the Dichroic PVA film. A wide variety ofglue compositions are available and is not particularly limited. Acommonly employed example is a water/alcohol solution containing adissolved polymer such as PVA or its derivatives and a boron compoundsuch as boric acid. Alternatively, the solution may be free orsubstantially free of dissolved polymer and comprise a reagent thatcrosslinks PVA. The reagent may crosslink PVA either ionically orcovalently or a combination of both types of reagents may be used.Appropriate crosslinking ions include but are not limited to cationssuch as calcium, magnesium, barium, strontium, boron, beryllium,aluminum, iron, copper, cobalt, lead, silver, zirconium and zinc ions.Boron compounds such as boric acid and zirconium compounds such aszirconium nitrate or zirconium carbonate are particularly preferred.Examples of covalent crosslinking reagents include polycarboxylic acidsor anhydrides; polyamines; epihalohydrins; diepoxides; dialdehydes;diols; carboxylic acid halides, ketenes and like compounds. The amountof the solution applied onto the films can vary widely depending on itscomposition. For example, a wet film coverage as low as 1 cc/m² and ashigh as 100 cc/m² are possible. Low wet film coverages are desirable toreduce the drying time needed.

Low birefringence protective polymer films suitable for use in thepresent invention comprise polymeric materials having low IntrinsicBirefringence Δn_(int) that form high clarity films with high lighttransmission (i.e., >85%). Preferably, the low birefringence protectivepolymer film has in-plane birefringence, Δn_(in) of less than about1×10⁻⁴ and an out-of-plane birefringence, Δn_(th) of from 0.005 to−0.005.

Exemplary polymeric materials for use in the low birefringenceprotective polymer films include cellulose esters (including triacetylcellulose (TAC), cellulose diacetate, cellulose acetate butyrate,cellulose acetate propionate), polycarbonates (such as LEXAN availablefrom General Electric Corp.,bisphenol-A-trimethylcyclohexane-polycarbonate,bisphenol-A-phthalate-polycarbonate), polysulfones (such as UDELavailable from Amoco Performance Products Inc.), polyacrylates, andcyclic olefin polymers (such as ARTON available from JSR Corp., ZEONEXand ZEONOR available from Nippon Zeon, TOPAS supplied by Ticona), amongothers. Preferably, the low birefringence protective polymer film of theinvention comprises TAC, polycarbonate, or cyclic olefin polymers duetheir commercial availability and excellent optical properties.

The low birefringence protective polymer film has a thickness from about5 to 200 micrometers, preferably from about 5 to 80 micrometers and mostpreferably from about 20 to 80 micrometers. Films having thickness of 20to 80 micrometers are most preferred due to cost, handling, and theability to fabricate thinner polarizer plates. In a preferred embodimentof the current invention, polarizer plates assembled from cover sheetsof the invention have a total thickness of less than 120 micrometers,and most preferably less than 80 micrometers.

The layer promoting adhesion to PVA preferably comprises a hydrophilicpolymer. Hydrophilic polymers suitable for the purpose of the presentinvention include both synthetic and natural polymers. Naturallyoccurring polymers include proteins, protein derivatives, cellulosederivatives (e.g. cellulose esters), polysaccharides, casein, and thelike, and synthetic polymers include poly(vinyl lactams), acrylamidepolymers, polyvinyl alcohol and its derivatives, hydrolyzed polyvinylacetates, polymers of alkyl and sulfoalkyl acrylates and methacrylates,polyamides, polyvinyl pyridine, acrylic acid polymers, maleic anhydridecopolymers' polyalkylene oxide, methacrylamide copolymers, polyvinyloxazolidinones, maleic acid copolymers, vinyl amine copolymers,methacrylic acid copolymers, acryloyloxyalkyl sulfonic acid copolymers,vinyl imidazole copolymers, vinyl sulfide copolymers, homopolymer orcopolymers containing styrene sulfonic acid, and the like.

Preferably, the hydrophilic polymer is water-soluble. The most preferredhydrophilic polymers are polyvinyl alcohol and its derivatives.Particularly preferred polyvinyl alcohol polymers have a degree ofhydrolysis of between 75 and 99.5% and have a weight average molecularweight of greater than 10,000.

In one embodiment, the layer promoting adhesion to polyvinyl alcoholfilms may further comprise hydrophobic polymer particles such as waterdispersible polymers and polymer latexes. Preferably these polymerparticles contain hydrogen-bonding accepting groups, which includeshydroxyl, carboxyl, amino, or sulfonyl moieties. Suitable polymerparticles comprise addition-type polymers and interpolymers preparedfrom ethylenically unsaturated monomers such as acrylates includingacrylic acid, methacrylates including methacrylic acid, acrylamides andmethacrylamides, itaconic acid and its half esters and diesters,styrenes including substituted styrenes, acrylonitrile andmethacrylonitrile, vinyl acetates, vinyl ethers, vinyl and vinylidenehalides, and olefins. In addition, crosslinking and graft-linkingmonomers such as 1,4-butyleneglycol methacrylate, trimethylolpropanetriacrylate, allyl methacrylate, diallyl phthalate, divinyl benzene, andthe like may be used. Other suitable polymer dispersions arepolyurethane dispersions or polyester ionomer dispersions,polyurethane/vinyl polymer dispersions, and fluoropolymer dispersions.Preferably, polymers for use in the polymer particles of the inventionhave a weight average molecular weight of greater than about 10,000 anda glass transition temperature (Tg) of less than about 25° C. Ingeneral, high molecular weight, low Tg polymer particles provideimproved adhesion of the layer to both PVA dichroic films and the tielayer.

These polymer particles can have a particle size in the range of from 10nanometers to 1 micron, preferably from 10 to 500 nanometers, and mostpreferably from 10 to 200 nanometers. Suitably, the polymer particlescan comprise between 5 and 40 weight % of the layer promoting adhesionto PVA in such an embodiment.

The layer promoting adhesion to PVA may also contain a crosslinkingagent. Crosslinking agents useful for the practice of the inventioninclude any compounds that are capable of reacting with reactivemoieties present on the water soluble polymer and/or polymer particles.Such crosslinking agents include aldehydes and related compounds,pyridiniums, olefins such as bis(vinylsulfonyl methyl)ether,carbodiimides, epoxides, triazines, polyfunctional aziridines,methoxyalkyl melamines, polyisocyanates, and the like. These compoundscan be readily prepared using the published synthetic procedure orroutine modifications that would be readily apparent to one skilled inthe art of synthetic organic chemistry. Additional crosslinking agentsthat may also be successfully employed in the layer promoting adhesionto PVA include multivalent metal ion such as zinc, calcium, zirconiumand titanium.

The layer promoting adhesion to PVA is typically applied at a driedcoating weight of 50 to 3000 mg/m², preferably 250 to 1000 mg/m². Thelayer is highly transparent and, preferably, has a light transmission ofgreater than 95%.

For the guarded cover sheet composites use in the invention, preferablythe layer promoting adhesion to PVA is on the same side of the lowbirefringence protective polymer film as the carrier substrate. Mostpreferably, the layer promoting adhesion to PVA is applied directly ontothe carrier substrate or onto a subbing layer on the carrier substrate.The layer promoting adhesion to PVA may be coated in a separate coatingapplication or it may be applied simultaneously with one or more otherlayers.

In order to provide good wetting by the water-based glues that may beemployed to laminate the cover sheets of the invention to PVA dichroicfilms, it is preferred that the PVA adhesion promoting layer has a watercontact angle of less than 20°. The adhesion promoting layer alsopreferably has a water swell of between 20 and 1000% to promote goodcontact and perhaps intermixing of the adhesion promoting layer with theglue and/or PVA dichroic film.

In one embodiment, an optional tie layer can comprises at least 50weight % of a polymer having an acid number of between 20 and 200 thatis soluble in organic solvent at 20° C. Preferably the acidfunctionality is a carboxylic acid. Polymer suitable for use in the tielayer include interpolymers of ethylenically unsaturated monomerscomprising carboxylic acid groups, acid-containing cellulosic polymerssuch as cellulose acid phthalate and cellulose acetate trimellitate,polyurethanes having carboxylic acid groups, and others. Suitableinterpolymers of ethylenically unsaturated monomers comprisingcarboxylic acid groups include acrylates including acrylic acid,methacrylates including methacrylic acid, acrylamides andmethacrylamides, itaconic acid and its half esters and diesters,styrenes including substituted styrenes, acrylonitrile andmethacrylonitrile, vinyl acetates, vinyl ethers, vinyl and vinylidenehalides, and olefins.

Organic solvents suitable for solubilizing and coating the tie layerpolymer include chlorinated solvents (methylene chloride and 1,2dichloroethane), alcohols (methanol, ethanol, n-propanol, isopropanol,n-butanol, isobutanol, diacetone alcohol and cyclohexanol), ketones(acetone, methylethyl ketone, methylisobutyl ketone, and cyclohexanone),esters (methyl acetate, ethyl acetate, n-propyl acetate, isopropylacetate, isobutyl acetate, n-butyl acetate, and methylacetoacetate),aromatics (toluene and xylenes) and ethers (1,3-dioxolane,1,2-dioxolane, 1,3-dioxane, 1,4-dioxane, and 1,5-dioxane). In someapplications, small amounts of water may be used. Normally, the coatingsolutions are prepared with a blend of the aforementioned solvents.Preferred primary solvents include methylene chloride, acetone, methylacetate, and 1,3-dioxolane. Preferred co-solvents for use with theprimary solvents include methanol, ethanol, n-butanol and water.Preferably, the tie layer polymer is applied from the same or at leastcompatible solvent mixture to the low birefringence protective polymer.

The tie layer may also contain a crosslinking agent. Crosslinking agentsuseful for the practice of the invention include any compounds that arecapable of reacting with reactive moieties present on the polymer,particularly carboxylic acid. Such crosslinking agents include aldehydesand related compounds, pyridiniums, olefins such as bis(vinylsulfonylmethyl)ether, carbodiimides, epoxides, triazines, polyfunctionalaziridines, methoxyalkyl melamines, polyisocyanates, and the like. Thesecompounds can be readily prepared using the published syntheticprocedure or routine modifications that would be readily apparent to oneskilled in the art of synthetic organic chemistry. Additionalcrosslinking agents that may also be successfully employed in the layerinclude multivalent metal ion such as zinc, calcium, zirconium andtitanium.

The optional tie layer is typically applied at a dried coating weight of50 to 5000 mg/m², preferably 500 to 5000 mg/m² and has a thickness ofpreferably 0.5 to 5 micrometers. The layer is highly transparent and,preferably, has a light transmission of greater than 95%.

The tie layer can be applied onto an already coated and dried layerpromoting adhesion to PVA. The tie layer may be coated in a separatecoating application or it may be applied simultaneously with one or moreother layers. Preferably, for best adherence, the tie layer is appliedsimultaneously with the low birefringence protective polymer layer.

Carrier substrates suitable for the use in the present invention includepolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polycarbonate, polystyrene, and other polymeric films. Additionalsubstrates may include paper, laminates of paper and polymeric films,glass, cloth, aluminum and other metal supports. Preferably, the carriersubstrate is a polyester film comprising polyethylene terephthalate(PET) or polyethylene naphthalate (PEN). The thickness of the carriersubstrate is about 20 to 200 micrometers, typically about 40 to 100micrometers. Thinner carrier substrates are desirable due to both costand the weight per roll of guarded cover sheet composite. However,carrier substrates less than about 20 micrometers may not providesufficient dimensional stability or protection for the cover sheet.

The carrier substrate may be coated with one or more subbing layers ormay be pretreated with electrical discharge devices to enhance thewetting of the substrate by coating solutions. Since the cover sheetmust ultimately be peeled from the carrier substrate the adhesionbetween cover sheet and substrate is an important consideration. Subbinglayers and electrical discharge devices may also be employed to modifythe adhesion of the cover sheet to the carrier substrate. Subbing layersmay therefore function as either primer layers to improve wetting orrelease layers to modify the adhesion of the cover sheet to thesubstrate. The carrier substrate may be coated with two subbing layers,the first layer acting as a primer layer to improve wetting and thesecond layer acting as a release layer. The thickness of the subbinglayer is typically 0.05 to 5 micrometers, preferably 0.1 to 1micrometers.

Cover sheet/substrate composites having poor adhesion might be prone toblister after application of a second or third wet coating in amulti-pass operation. To avoid blister defects, adhesion should begreater than about 0.3 N/m between the first-pass layer of the coversheet and the carrier substrate. As already mentioned, the level ofadhesion may be modified by a variety of web treatments includingvarious subbing layers and various electronic discharge treatments.However, excessive adhesion between the cover sheet and substrate isalso undesirable since the cover sheet may be damaged during subsequentpeeling operations. In particular, cover sheet/substrate compositeshaving too great an adhesive force may peel poorly. The maximum adhesiveforce that allows acceptable peel behavior is dependent on the thicknessand tensile properties of the cover sheet. Typically, an adhesive forcebetween the cover sheet and the substrate greater than about 300 N/m maypeel poorly. Cover sheets peeled from such excessively well-adheredcomposites exhibit defects due to tearing of the cover sheet and/or dueto cohesive failure within the sheet. In a preferred embodiment of thepresent invention, the adhesion between the cover sheet and the carriersubstrate is less than 250 N/m. Most preferably, the adhesion betweenthe cover sheet and the carrier substrate is between 0.5 and 25 N/m.

In a preferred embodiment of the invention, the carrier substrate is apolyethylene terephthalate film having a first subbing layer (primerlayer) comprising a vinylidene chloride copolymer and second subbinglayer (release layer) comprising polyvinyl butyral. In another preferredembodiment of the invention the carrier substrate is polyethyleneterephthalate film that has been pretreated with a corona dischargeprior to application of the cover sheet.

Substrates may also have functional layers such as antistatic layerscontaining various polymer binders and conductive addenda in order tocontrol static charging and dirt and dust attraction. The antistaticlayer may be on either side of the carrier substrate, preferably it ison the side of the carrier substrate opposite to the cover sheet.

On the side of the substrate opposite to the cover sheet a backing layermay also be employed in order to provide a surface having appropriateroughness and coefficient of friction for good winding and conveyancecharacteristics. In particular, the backing layer comprises a polymericbinder such as a polyurethane or acrylic polymer containing mattingagent such a silica or polymeric beads. The matting agent helps toprevent the sticking of the front side of the guarded cover sheetcomposite to the backside during shipping and storage. The backing layermay also comprise a lubricant to provide a coefficient of friction ofabout 0.2 to 0.4. Typical lubricants include for example (1) liquidparaffin and paraffin or wax like materials such as carnauba wax,natural and synthetic waxes, petroleum waxes, mineral waxes and thelike; (2) higher fatty acids and derivatives, higher alcohols andderivatives, metal salts of higher fatty acids, higher fatty acidesters, higher fatty acid amides, polyhydric alcohol esters of higherfatty acids, etc., disclosed in U.S. Pat. Nos. 2,454,043; 2,732,305;2,976,148; 3,206,311; 3,933,516; 2,588,765; 3,121,060; 3,502,473;3,042,222; and 4,427,964, in British Patents 1,263,722; 1,198,387;1,430,997; 1,466,304; 1,320,757; 1,320,565; and 1,320,756; and in GermanPatents 1,284,295 and 1,284,294; (3) perfluoro- or fluoro- orfluorochloro-containing materials, which includepoly(tetrafluoroethylene), poly(trifluorochloroethylene),poly(vinylidene fluoride, poly(trifluorochloroethylene-co-vinylchloride), poly(meth)acrylates or poly(meth)acrylamides containingperfluoroalkyl side groups, and the like. However for lasting lubricitya polymerizable lubricant such as ADDITIVE 31, a methacryloxy-functionalsilicone polyether copolymer (from Dow Corning Corp.) is preferred.

In another embodiment, the guarded cover sheet composite comprises astrippable, protection layer on the surface of the cover sheet oppositeto the carrier substrate. The strippable, protection layer may beapplied by coating the layer or it may be applied by adhesively adheringor by electrostatically adhering, a preformed protection layer.Preferably, the protection layer is a transparent polymer layer. In oneparticular embodiment, the protection layer is a low birefringence layerthat allows optical inspection of the cover sheet without the need toremove the protection layer. Particularly useful polymers for use in theprotection layer include: cellulose esters, acrylics, polyurethanes,polyesters, cyclic olefin polymers, polystyrene, polyvinyl butyral,polycarbonate, and others. When a preformed protection layer is used, itis preferably a layer of polyester, polystyrene, or polyolefin film.

The strippable, protection layer is typically 5 to 100 micrometers inthickness. Preferably, the protection layer is 20 to 50 micrometersthick to insure adequate resistance to scratch and abrasion and provideeasy handling during removal of the protection layer.

When the strippable, protection layer is applied by coating methods itmay be applied to an already coated and dried cover sheet or theprotection layer may be coated simultaneously with one or more layerscomprising the cover sheet.

When the strippable, protection layer is a preformed layer it may have apressure sensitive adhesive layer on one surface that allows theprotection layer to be adhesively laminated to the guarded cover sheetcomposite using conventional lamination techniques. Alternatively, thepreformed protection layer may be applied by generating an electrostaticcharge on a surface of the cover sheet or the preformed protection layerand then bringing the two materials into contact in a roller nip. Theelectrostatic charge may be generated by any known electric chargegenerator, e.g., a corona charger, a tribocharger, conducting highpotential roll charge generator or contact charger, a static chargegenerator, and the like. The cover sheet or the preformed protectionlayer may be charged with a DC charge or a DC charge followed by an ACcharge in order to create an adequate level of charge adhesion betweenthe two surfaces. The level of electrostatic charge applied to provide asufficient bond between the cover sheet and the preformed protectionlayer is at least more than 50 volts, preferably at least more than 200volts. The charged surface of the cover sheet or the protection layerhas a resistivity of at least about 10¹²Ω/square, preferably at leastabout 10¹⁶Ω/square in order to insure that the electrostatic charge islong lasting.

As mentioned above, each cover sheet may have various auxiliary layersthat are necessary to improve the performance of the Liquid CrystalDisplay. Useful auxiliary layers that may be employed in the coversheets used in the invention include: abrasion resistant hardcoat layer,antiglare layer, anti-smudge layer or stain-resistant layer,antireflection layer, low reflection layer, antistatic layer, viewingangle compensation layer, and moisture barrier layer. Typically, thecover sheet closest to the viewer contains one or more of the followingauxiliary layers: the abrasion resistant layer, anti-smudge orstain-resistant layer, antireflection layer, and antiglare layer. One orboth of the cover sheets closest to the liquid crystal cell typicallycontain a viewing angle compensation layer. Any or all of the four coversheets employed in the LCD may optionally contain an antistatic layerand a moisture barrier layer.

The cover sheets may contain an abrasion resistant layer on the oppositeside of the low birefringence protective polymer film to the layerpromoting adhesion to PVA.

Particularly effective abrasion resistant layers comprise radiation orthermally cured compositions, and preferably the composition isradiation cured. Ultraviolet (UV) radiation and electron beam radiationare the most commonly employed radiation curing methods. UV curablecompositions are particularly useful for creating the abrasion resistantlayer of this invention and may be cured using two major types of curingchemistries, free radical chemistry and cationic chemistry. Acrylatemonomers (reactive diluents) and oligomers (reactive resins andlacquers) are the primary components of the free radical basedformulations, giving the cured coating most of its physicalcharacteristics. Photo-initiators are required to absorb the UV lightenergy, decompose to form free radicals, and attack the acrylate groupC═C double bond to initiate polymerization. Cationic chemistry utilizescycloaliphatic epoxy resins and vinyl ether monomers as the primarycomponents. Photo-initiators absorb the UV light to form a Lewis acid,which attacks the epoxy ring initiating polymerization. By UV curing ismeant ultraviolet curing and involves the use of UV radiation ofwavelengths between 280 and 420 nm preferably between 320 and 410 nm.

Examples of UV radiation curable resins and lacquers usable for anabrasion layer are those derived from photo polymerizable monomers andoligomers such as acrylate and methacrylate oligomers (the term“(meth)acrylate” used herein refers to acrylate and methacrylate), ofpolyfunctional compounds, such as polyhydric alcohols and theirderivatives having (meth)acrylate functional groups such as ethoxylatedtrimethylolpropane tri(meth)acrylate, tripropylene glycoldi(meth)acrylate, trimethylolpropane tri(meth)acrylate, diethyleneglycol di(meth)acrylate, pentaerythritol tetra(meth)acrylate,pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate,1,6-hexanediol di(meth)acrylate, or neopentyl glycol di(meth)acrylateand mixtures thereof, and acrylate and methacrylate oligomers derivedfrom low-molecular weight polyester resin, polyether resin, epoxy resin,polyurethane resin, alkyd resin, spiroacetal resin, epoxy acrylates,polybutadiene resin, and polythiol-polyene resin, and the like andmixtures thereof, and ionizing radiation-curable resins containing arelatively large amount of a reactive diluent. Reactive diluents usableherein include monofunctional monomers, such as ethyl(meth)acrylate,ethylhexyl (meth)acrylate, styrene, vinyltoluene, andN-vinylpyrrolidone, and polyfunctional monomers, for example,trimethylolpropane tri(meth)acrylate, hexanediol (meth)acrylate,tripropylene glycol di(meth)acrylate, diethylene glycoldi(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritolhexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, or neopentyl glycoldi(meth)acrylate.

Among others, conveniently used radiation curable lacquers includeurethane (meth)acrylate oligomers. These are derived from reactingdiisocyanates with a oligo(poly)ester or oligo(poly)ether polyol toyield an isocyanate terminated urethane. Subsequently, hydroxyterminated acrylates are reacted with the terminal isocyanate groups.This acrylation provides the unsaturation to the ends of the oligomer.The aliphatic or aromatic nature of the urethane acrylate is determinedby the choice of diisocyanates. An aromatic diisocyanate, such astoluene diisocyanate, will yield an aromatic urethane acrylate oligomer.An aliphatic urethane acrylate will result from the selection of analiphatic diisocyanate, such as isophorone diisocyanate or hexyl methyldiisocyanate. Beyond the choice of isocyanate, polyol backbone plays apivotal role in determining the performance of the final the oligomer.Polyols are generally classified as esters, ethers, or a combination ofthese two. The oligomer backbone is terminated by two or more acrylateor methacrylate units, which serve as reactive sites for free radicalinitiated polymerization. Choices among isocyanates, polyols, andacrylate or methacrylate termination units allow considerable latitudein the development of urethane acrylate oligomers. Urethane acrylates,like most oligomers, are typically high in molecular weight andviscosity. These oligomers are multifunctional and contain multiplereactive sites. Because of the increased number of reactive sites, thecure rate is improved and the final product is cross-linked. Theoligomer functionality can vary from 2 to 6.

Among others, conveniently used radiation curable resins includepolyfunctional acrylic compounds derived from polyhydric alcohols andtheir derivatives such as mixtures of acrylate derivatives ofpentaerythritol such as pentaerythritol tetraacrylate andpentaerythritol triacrylate functionalized aliphatic urethanes derivedfrom isophorone diisocyanate. Some examples of urethane acrylateoligomers used in the practice of this invention that are commerciallyavailable include oligomers from Sartomer Company (Exton, Pa.). Anexample of a resin that is conveniently used in the practice of thisinvention is CN 968 from Sartomer Company.

A photo-polymerization initiator, such as an acetophenone compound, abenzophenone compound, Michler's benzoyl benzoate, α-amyloxime ester, ora thioxanthone compound and a photosensitizer such as n-butyl amine,triethylamine, or tri-n-butyl phosphine, or a mixture thereof isincorporated in the ultraviolet radiation curing composition. In thepresent invention, conveniently used initiators are 1-hydroxycyclohexylphenyl ketone and 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropanone-1.

The abrasion resistant layer is typically applied after coating anddrying the low birefringence protective polymer film. The abrasionresistant layer of this invention is applied as a coating compositionthat typically also includes organic solvents. Preferably theconcentration of organic solvent is 1-99% by weight of the total coatingcomposition.

Examples of solvents employable for coating the abrasion resistant layerinclude solvents such as methanol, ethanol, propanol, butanol,cyclohexane, heptane, toluene and xylene, esters such as methyl acetate,ethyl acetate, propyl acetate and mixtures thereof. With the properchoice of solvent, adhesion of the abrasion resistant layer can beimproved while minimizing migration of plasticizers and other addendafrom the low birefringence protective polymer film, enabling thehardness of the abrasion resistant layer to be maintained. Suitablesolvents for TAC low birefringence protective polymer film are aromatichydrocarbon and ester solvents such as toluene and propyl acetate.

The UV polymerizable monomers and oligomers are coated and dried, andsubsequently exposed to UV radiation to form an optically clearcross-linked abrasion resistant layer. The preferred UV cure dosage isbetween 50 and 1000 mJ/cm².

The thickness of the optional abrasion resistant layer is generallyabout 0.5 to 50 micrometers preferably 1 to 20 micrometers, morepreferably 2 to 10 micrometers.

The abrasion resistant layer is preferably colorless, but it isspecifically contemplated that this layer can have some color for thepurposes of color correction, or for special effects, so long as it doesnot detrimentally affect the formation or viewing of the display throughthe overcoat. Thus, there can be incorporated into the polymer dyes thatwill impart color. In addition, additives can be incorporated into thepolymer that will give to the layer desired properties. Other additionalcompounds may be added to the coating composition, includingsurfactants, emulsifiers, coating aids, lubricants, matte particles,rheology modifiers, crosslinking agents, antifoggants, inorganic fillerssuch as conductive and nonconductive metal oxide particles, pigments,magnetic particles, biocide, and the like.

The abrasion resistant layer typically provides a layer having a pencilhardness (using the Standard Test Method for Hardness by Pencil TestASTM D3363) of at least 2H and preferably 2H to 8H.

The cover sheets used in the invention may contain an antiglare layer, alow reflection layer or an antireflection layer on the same side of thecarrier substrate as the low birefringence protective polymer film. Theantiglare layer, low reflection layer or antireflection layer is locatedon the opposite side of the low birefringence protective polymer film tothe layer promoting adhesion to PVA. Such layers are employed in an LCDin order to improve the viewing characteristics of the display,particularly when it is viewed in bright ambient light. The refractiveindex of an abrasion resistant, hard coat is about 1.50, while the indexof the surrounding air is 1.00. This difference in refractive indexproduces a reflection from the surface of about 4%.

An antiglare coating provides a roughened or textured surface that isused to reduce specular reflection. All of the unwanted reflected lightis still present, but it is scattered rather than specularly reflected.For the purpose of the present invention, the antiglare coatingpreferably comprises a radiation cured composition that has a texturedor roughened surface obtained by the addition of organic or inorganic(matting) particles or by embossing the surface. The radiation curedcompositions described hereinabove for the abrasion resistant layer arealso effectively employed in the antiglare layer. Surface roughness ispreferably obtained by the addition of matting particles to theradiation cured composition. Suitable particles include inorganiccompounds having an oxide, nitride, sulfide or halide of a metal, metaloxides being particularly preferred. As the metal atom, Na, K, Mg, Ca,Ba, Al, Zn, Fe, Cu, Ti, Sn, In, W, Y, Sb, Mn, Ga, V, Nb, Ta, Ag, Si, B,Bi, Mo, Ce, Cd, Be, Pb and Ni are suitable, and Mg, Ca, B and Si aremore preferable. An inorganic compound containing two types of metal mayalso be used. A particularly preferable inorganic compound is silicondioxide, namely silica.

Additional particles suitable for use in the optional antiglare layerinclude the layered clays described in commonly-assigned U.S. patentapplication Ser. No. 10/690,123, filed Oct. 21, 2003. The most suitablelayered particles include materials in the shape of plates with highaspect ratio, which is the ratio of a long direction to a shortdirection in an asymmetric particle. Preferred layered particles arenatural clays, especially natural smectite clay such as montmorillonite,nontronite, beidellite, volkonskoite, hectorite, saponite, sauconite,sobockite, stevensite, svinfordite, halloysite, magadiite, kenyaite andvermiculite as well as layered double hydroxides or hydrotalcites. Mostpreferred clay materials include natural montrnorillonite, hectorite andhydrotalcites, because of commercial availability of these materials.

Suitable layered materials may comprise phyllosilicates, for example,montmorillonite, particularly sodium montmorillonite, magnesiummontmorillonite, and/or calcium montmorillonite, nontronite, beidellite,volkonskoite, hectorite, saponite, sauconite, sobockite, stevensite,svinfordite, vermiculite, magadiite, kenyaite, talc, mica, kaolinite,and mixtures thereof. Other useful layered materials may include illite,mixed layered illite/smectite minerals, such as ledikite and admixturesof illites with the layered materials named above. Other useful layeredmaterials, particularly useful with anionic matrix polymers, may includethe layered double hydroxide clays or hydrotalcites, such asMg₆Al_(3.4)(OH)_(18.8)(CO₃)_(1.7)H₂O, which have positively chargedlayers and exchangeable anions in the interlayer spaces. Preferredlayered materials are swellable so that other agents, usually organicions or molecules, may splay, that is, intercalate and/or exfoliate, thelayered material resulting in a desirable dispersion of the inorganicphase. These swellable layered materials include phyllosilicates of the2:1 type, as defined in the literature (for example, “An introduction toclay colloid chemistry,” by H. van Olphen, John Wiley & SonsPublishers). Typical phyllosilicates with ion exchange capacity of 50 to300 milliequivalents per 100 grams are preferred. Generally, it isdesirable to treat the selected clay material to separate theagglomerates of platelet particles to small crystals, also calledtactoids, prior to introducing the platelet particles to the antiglarecoating. Predispersing or separating the platelet particles alsoimproves the binder/platelet interface. Any treatment that achieves theabove goals may be used. Examples of useful treatments includeintercalation with water soluble or water insoluble polymers, organicreagents or monomers, silane compounds, metals or organometallics,organic cations to effect cation exchange, and their combinations.

Additional particles for use in the optional antiglare layer includepolymer matte particles or beads which are well known in the art. Thepolymer particles may be solid or porous, preferably they arecrosslinked polymer particles. Porous polymer particles for use in anantiglare layer are described in commonly-assigned U.S. patentapplication Ser. No. 10/715,706, filed Nov. 18, 2003.

Particles for use in the antiglare layer have an average particle sizeranging from 2 to 20 micrometers, preferably from 2 to 15 micrometersand most preferably from 4 to 10 micrometers. They are present in thelayer in an amount of at least 2 wt percent and less than 50 percent,typically from about 2 to 40 wt. percent, preferably from 2 to 20percent and most preferably from 2 to 10 percent.

The thickness of the antiglare layer is generally about 0.5 to 50micrometers preferably 1 to 20 micrometers more preferably 2 to 10micrometers.

Preferably, the antiglare layer has a 60° Gloss value, according to ASTMD523, of less than 100, preferably less than 90 and a transmission hazevalue, according to ASTM D-1003 and JIS K-7105 methods, of less than50%, preferably less than 30%.

In another embodiment, a low reflection layer or antireflection layer isused in combination with an abrasion resistant hard coat layer orantiglare layer. The low reflection or antireflection coating is appliedon top of the abrasion resistant or antiglare layer. Typically, a lowreflection layer provides an average specular reflectance (as measuredby a spectrophotometer and averaged over the wavelength range of 450 to650 nm) of less than 2%. Antireflection layers provide average specularreflectance values of less than 1%.

Suitable low reflection layers for the cover sheet comprisefluorine-containing homopolymers or copolymers having a refractive indexof less than 1.48, preferably with a refractive index between about 1.35and 1.40. Suitable fluorine-containing homopolymers and copolymersinclude: fluoro-olefins (for example, fluoroethylene, vinylidenefluoride, tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene,perfluoro-2,2-dimethyl-1,3-dioxol), partially or completely fluorinatedalkyl ester derivatives of (meth)acrylic acid, and completely orpartially fluorinated vinyl ethers, and the like. The effectiveness ofthe layer may be improved by the incorporation of submicron-sizedinorganic particles or polymer particles that induce interstitial airvoids within the coating. This technique is further described in U.S.Pat. No. 6,210,858 and U.S. Pat. No. 5,919,555. Further improvement ofthe effectiveness of the low reflection layer may be realized with therestriction of air voids to the internal particle space ofsubmicron-sized polymer particles with reduced coating haze penalty, asdescribed in commonly-assigned U.S. patent application Ser. No.10/715,655, filed Nov. 18, 2003.

The thickness of the optional low reflection layer is 0.01 to 1micrometer and preferably 0.05 to 0.2 micrometer.

An antireflection layer may comprise a monolayer or a multi-layer.Antireflection layers comprising a monolayer typically providereflectance values less than 1% at only a single wavelength (within thebroader range of 450 to 650 nm). A commonly employed monolayerantireflection coating that is suitable for use in the present inventioncomprises a layer of a metal fluoride such as magnesium fluoride (MgF₂).The layer may be applied by well-known vacuum deposition technique or bya sol-gel technique. Typically, such a layer has an optical thickness(i.e., the product of refractive index of the layer times layerthickness) of approximately one quarter-wavelength at the wavelengthwhere a reflectance minimum is desired.

Although a monolayer can effectively reduce the reflection of lightwithin a very narrow wavelength range, more often a multi-layercomprising several (typically, metal oxide based) transparent layerssuperimposed on one another is used to reduce reflection over a widewavelength region (i.e., broadband reflection control). For such astructure, half wavelength layers are alternated with quarter wavelengthlayers to improve performance. The multi-layer antireflection coatingmay comprise two, three, four, or even more layers. Formation of thismulti-layer typically requires a complicated process comprising a numberof vapor deposition procedures or sol-gel coatings, which correspond tothe number of layers, each layer having a predetermined refractive indexand thickness. Precise control of the thickness of each layer isrequired for these interference layers. The design of suitablemulti-layer antireflection coatings for use in the present invention iswell known in the patent art and technical literature, as well as beingdescribed in various textbooks, for example, in H. A. Macleod, “ThinFilm Optical Filters,” Adam Hilger, Ltd., Bristol 1985 and James D.Rancourt, “Optical Thin Films User's Handbook”, Macmillan PublishingCompany, 1987.

The cover sheets used in the invention may also contain a moisturebarrier layer. The moisture barrier layer typically comprises ahydrophobic polymer such as a vinylidene chloride polymer, vinylidenefluoride polymer, polyurethane, polyolefin, fluorinated polyolefin,polycarbonate, and others, having a low moisture permeability.Preferably, the hydrophobic polymer comprises vinylidene chloride. Morepreferably, the hydrophobic polymer comprises 70 to 99 weight percent ofvinylidene chloride. The moisture barrier layer may be applied byapplication of an organic solvent-based or aqueous coating formulation.To provide effective moisture barrier properties the layer should be atleast 1 micrometer in thickness, preferably from 1 to 10 micrometers inthickness, and most preferably from 2 to 8 micrometers in thickness. Thecover sheet of the invention comprising a moisture barrier layer has amoisture vapor transmission rate (MVTR) according to ASTM F-1249 that isless than 1000 g/m²/day, preferably less than 800 g/m²/day and mostpreferably less than 500 g/m²/day. The use of such a barrier layer inthe cover sheet of the invention provides improved resistance to changesin humidity and increased durability of the polarizer plate comprisingthe cover sheet, especially for TAC cover sheets having a thickness lessthan about 40 micrometers.

The cover sheets used in the invention may contain a transparentantistatic layer. The antistatic layer aids in the control of staticcharging that may occur during the manufacture and use of the coversheet composite. Effective control of static charging reduces thepropensity for the attraction of dirt and dust to the cover sheetcomposite. The guarded cover sheet composite may be particularly proneto triboelectric charging during the peeling of the cover sheet from thecarrier substrate. The so-called “separation charge” that results fromthe separation of the cover sheet and the substrate can be effectivelycontrolled by an antistatic layer having a resistivity of less thanabout 1×10¹¹Ω/square, preferably less than 1×10¹⁰Ω/square, and mostpreferably less than 1×10⁹Ω/square.

Various polymeric binders and conductive materials may be employed inthe antistatic layer. Polymeric binders useful in the antistatic layerinclude any of the polymers commonly used in the coating art, forexample, interpolymers of ethylenically unsaturated monomers, cellulosederivatives, polyurethanes, polyesters, hydrophilic colloids such asgelatin, polyvinyl alcohol, polyvinyl pyrrolidone, and others.

Conductive materials employed in the antistatic layer may be eitherionically-conductive or electronically-conductive. Ionically-conductivematerials include simple inorganic salts, alkali metal salts ofsurfactants, polymeric electrolytes containing alkali metal salts, andcolloidal metal oxide sols (stabilized by metal salts). Of these,ionically-conductive polymers such as anionic alkali metal salts ofstyrene sulfonic acid copolymers and cationic quaternary ammoniumpolymers of U.S. Pat. No. 4,070,189 and ionically-conductive colloidalmetal oxide sols which include silica, tin oxide, titania, antimonyoxide, zirconium oxide, alumina-coated silica, alumina, boehmite, andsmectite clays are preferred.

The optional antistatic layer preferably contains anelectronically-conductive material due to their humidity and temperatureindependent conductivity. Suitable materials include:

1) electronically-conductive metal-containing particles includingdonor-doped metal oxides, metal oxides containing oxygen deficiencies,and conductive nitrides, carbides, and bromides. Specific examples ofparticularly useful particles include conductive SnO₂, In₂O, ZnSb₂O₆,InSbO₄, TiB₂, ZrB₂, NbB₂, TaB₂, CrB, MoB, WB, LaB₆, ZrN, TiN, WC, HfC,HfN, and ZrC. Examples of the patents describing these electricallyconductive particles include; U.S. Pat. Nos. 4,275,103; 4,394,441;4,416,963; 4,418, 141; 4,431,764; 4,495,276; 4,571,361; 4,999,276;5,122,445; and 5,368, 995;

2) fibrous electronic conductive particles comprising, for example,antimony-doped tin oxide coated onto non-conductive potassium titanatewhiskers as described in U.S. Pat. Nos. 4,845,369 and 5,166,666,antimony-doped tin oxide fibers or whiskers as described in U.S. Pat.Nos. 5,719,016 and 5,0731,119, and the silver-doped vanadium pentoxidefibers described in U.S. Pat. Nos. 4,203,769; and

3) electronically-conductive polyacetylenes, polythiophenes, andpolypyrroles, preferably the polyethylene dioxythiophene described inU.S. Pat. No. 5,370,981 and commercially available from Bayer Corp. asBAYTRON P.

The amount of the conductive agent used in the antistatic layer can varywidely depending on the conductive agent employed. For example, usefulamounts range from about 0.5 mg/m² to about 1000 mg/m², preferably fromabout 1 mg/m² to about 500 mg/m². The antistatic layer has a thicknessof from 0.05 to 5 micrometers, preferably from 0.1 to 0.5 micrometers toinsure high transparency.

Since contrast, color reproduction, and stable gray scale intensitiesare important quality attributes for electronic displays that employliquid crystal technology. The primary factor limiting the contrast of aliquid crystal display is the propensity for light to “leak” throughliquid crystal elements or cells, which are in the dark or “black” pixelstate. Furthermore, the leakage and hence contrast of a liquid crystaldisplay are also dependent on the direction from which the displayscreen is viewed. Typically the optimum contrast is observed only withina narrow viewing angle range centered about the normal incidence to thedisplay and falls off rapidly as the viewing direction deviates from thedisplay normal. In color displays, the leakage problem not only degradesthe contrast but also causes color or hue shifts with an associateddegradation of color reproduction.

Thus, one of the major factors measuring the quality of LCDs is theviewing angle characteristic, which describes a change in contrast ratiofrom different viewing angles. It is desirable to be able to see thesame image from a wide variation in viewing angles and this ability hasbeen a shortcoming with liquid crystal display devices. One way toimprove the viewing angle characteristic is to employ a cover sheethaving a viewing angle compensation layer (also referred to as acompensation layer, retarder layer, or phase difference layer), withproper optical properties, between the Dichroic PVA film and liquidcrystal cell, such as disclosed in U.S. Pat. Nos. 5,583,679; 5,853,801;5,619,352; 5,978,055; and 6,160,597. A compensation film according toU.S. Pat. Nos. 5,583,679 and 5,853,801 based on discotic liquid crystalswhich have negative birefringence, is widely used.

Viewing angle compensation layers for use in the cover sheets used inthe present invention are optically anisotropic layers. The opticallyanisotropic, viewing angle compensation layers may comprise positivelybirefringent materials or negatively birefringent materials. Thecompensation layer may be optically uniaxial or optically biaxial. Thecompensation layer may have its optic axis tilted in the planeperpendicular to the layer. The tilt of the optic axis may be constantin the layer thickness direction or the tilt of the optic axis may varyin the layer thickness direction.

Optically anisotropic, viewing angle compensation layers may comprisethe negatively birefringent, discotic liquid crystals described in U.S.Pat. Nos. 5,583,679 and 5,853,801; the positively birefringent nematicliquid crystals described in U.S. Pat. No. 6,160,597; the negativelybirefringent amorphous polymers described in commonly assigned U.S.Patent Application Publication 2004/0021814A and U.S. patent applicationSer. No. 10/745,109, filed Dec. 23, 2003. These latter two patentapplications describe compensation layers comprising polymers thatcontain non-visible chromophore groups such as vinyl, carbonyl, amide,imide, ester, carbonate, sulfone, azo, and aromatic groups (i.e.benzene, naphthalate, biphenyl, bisphenol A) in the polymer backbone andthat preferably have a glass transition temperature of greater than 180degree C. Such polymers are particularly useful in the compensationlayer of the present invention. Such polymers include polyesters,polycarbonates, polyimides, polyetherimides, and polythiophenes. Ofthese, particularly preferred polymers for use in the present inventioninclude: (1) a poly(4,4′-hexafluoroisopropylidene-bisphenol)terephthalate-co-isophthalate; (2) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene bisphenol) terephthalate;(3) a poly(4,4′-isopropylidene-2,2′6,6′-tetrachlorobisphenol)terephthalate-co-isophthalate; (4) apoly(4,4′-hexafluoroisopropylidene)-bisphenol-co-(2-norbornylidene)-bisphenolterephthalate; (5) apoly(4,4′-hexahydro-4,7-methanoindan-5-ylidene)-bisphenol-co-(4,4′-isopropylidene-2,2′,6,6′-tetrabromo)-bisphenolterephthalate; (6) apoly(4,4′-isopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol)terephthalate-co-isophthalate; (7) apoly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol)terephthalate-co-isophthalate; or (8) copolymers of any two or more ofthe foregoing. A compensation layer comprising these polymers typicallyhas an out-of-plane retardation, R_(th), that is more negative than −20nm, preferably R_(th) is from −60 to −600 nm, and most preferably R_(th)is from −150 to −500 nm.

Another optional compensation layer suitable cover sheets used in thepresent invention includes an optically anisotropic layer comprising anexfoliated inorganic clay material in a polymeric binder as described inJapanese Patent Application 11095208A.

The auxiliary layers of the invention can be applied by any of a numberof well known liquid coating techniques, such as dip coating, rodcoating, blade coating, air knife coating, gravure coating, microgravurecoating, reverse roll coating, slot coating, extrusion coating, slidecoating, curtain coating, or by vacuum deposition techniques. In thecase of liquid coating, the wet layer is generally dried by simpleevaporation, which may be accelerated by known techniques such asconvection heating. The auxiliary layer may be applied simultaneouslywith other layers such as subbing layers and the low birefringenceprotective polymer film. Several different auxiliary layers may becoated simultaneously using slide coating, for example, an antistaticlayer may be coated simultaneously with a moisture barrier layer or amoisture barrier layer may be coated simultaneously with a viewing anglecompensation layer. Known coating and drying methods are described infurther detail in Research Disclosure 308119, Published December 1989,pages 1007 to 1008.

The cover sheets used in the invention are suitable for use with a widevariety of LCD display modes, for example, Twisted Nematic (TN), SuperTwisted Nematic (STN), Optically Compensated Bend (OCB), In PlaneSwitching (IPS), or Vertically Aligned (VA) liquid crystal displays.These various liquid crystal display technologies have been reviewed inU.S. Pat. No. 5,619,352 (Koch et al.), U.S. Pat. No. 5,410,422 (Bos),and U.S. Pat. No. 4,701,028 (Clerc et al.).

FIG. 8 presents a cross-sectional illustration showing a liquid crystalcell 260 having polarizer plates 252 and 254 disposed on either side.Polarizer plate 254 is on the side of the LCD cell closest to theviewer. Each polarizer plate employs two cover sheets. For the purposeof illustration, polarizer plate 254 is shown with an uppermost coversheet (this is the cover sheet closest to the viewer) comprising a layerpromoting adhesion to PVA 261, tie layer 262, low birefringenceprotective polymer film 264, barrier layer 266, and antiglare layer 268.The lowermost cover sheet contained in polarizer plate 254 comprises alayer promoting adhesion to PVA 261, tie layer 262, low birefringenceprotective polymer film 264, barrier layer 266, and viewing anglecompensation layer 272. On the opposite side of the LCD cell, polarizerplate 252 is shown with an uppermost cover sheet, which for the purposeof illustration, comprises a layer promoting adhesion to PVA 261, tielayer 262, low birefringence protective polymer film 264, barrier layer266, and viewing angle compensation layer 272. Polarizer plate 252 alsohas a lowermost cover sheet comprising a layer promoting adhesion to PVA261, tie layer 262, low birefringence protective polymer film 264, andbarrier layer 266.

The present invention is illustrated in more detail by the followingnon-limiting examples.

EXAMPLES Example 1

A 100 micrometer thick poly(ethylene terephthalate) (PET) carriersubstrate having an antistatic backing layer (backside) is coated on itsfront surface with a layer promoting adhesion to PVA film comprisingCELVOL 205 PVA (polyvinyl alcohol having a degree of hydrolysis of about88-89%, available from Celanese Corp.) having a dry coating weight ofabout 750 mg/m², and NEOREZ R-600 (polyurethane dispersion fromNeoResins Inc.) having a coating weight of about 250 mg/m². The driedlayer is then overcoated with a triacetyl cellulose (TAC) formulationcomprising three layers: a surface layer comprising CA-438-80S(triacetyl cellulose from Eastman Chemical) having a dry coating weightof about 2080 mg/m², diethyl phthalate having a dry coating weight ofabout 208 mg/m², and SURFLON S-8405-S50 (a fluorinated surfactant fromSemi Chemical Co. Ltd) having a dry coating weight of about 210 mg/m²; amid layer comprising CA-438-80S having a dry coating weight of about18990 mg/m², Surflon® S-8405-S50 having a dry coating weight of about295 mg/m², diethyl phthalate having a dry coating weight of about 1900mg/m², TINUVIN® 8515 UV absorber (a mixture of2-(2′-Hydroxy-3′-tert-butyl-5′-methylphenyl)-5-chloro benzotriazole and2-(2′-Hydroxy-3′,5′-ditert-butylphenyl)-benzotriazole, available fromCiba Specialty Chemicals) having a dry coating weight of about 840mg/m², and PARSOL 1789 UV absorber(4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane, available from RocheVitamins Inc.) having a dry coating weight of about 8.4 mg/ft²; a lowerlayer as the tie layer comprising a mixture of 95:5 cellulose acetatetrimellitate (Sigma-Aldrich) and trimethyl borate and having a drycoating weight of about 1000 mg/m². The TAC formulation was applied witha multi-slot slide hopper using a mixture of methylene chloride andmethanol as the coating solvent. The cellulose acetate trimellitate hasan acid number of 182.

The guarded cover sheet composite made above, 1330 mm in width, is woundonto a supply roll. The outer diameter of the supply roll after windingis 300 mm on a 152 mm core. In accordance with the present invention,the dried TAC coating is peeled off from the PET carrier substrate atthe interface between the front side of the carrier substrate and thelayer promoting adhesion of PVA film. The peeled film is then laminatedto a PVA film having a thickness of about 25 micrometers using a gluesolution comprising 61.5% water, 38.3% methanol, 0.13% boric acid, and0.07% zinc chloride. The laminated film is dried in an oven at 60° C.for about 3 minutes.

In accordance with the present process, continuous production, includinglamination, occurs during roll change. The machine line speed ismaintained at 3 meters per minute. The unwind tension of the supply rollis held constant at 10 to 150 Newtons per meter width, controlled by itsaccumulator position. The PET winding roll has a core outer diameter of152 mm. The PET winders are controlled by its accumulator position; itstension adjusted by accumulator air cylinder pressure. The PET windingtension is 300 to 400 Newtons per meter width at constant tension(alternatively taper tension may be used). A 125 mm diameter roller isused as the peeling station, where the PET carrier web touches thepeeling roller. The unguarded cover sheet between the peeling stationand the lamination nip is controlled to a tension of 100 to 500 Newtonsper meter width. The lamination nip force is set to 100 to 500 Newtonsper meter width. After peeled, the PET carrier web tension is 300 to 400Newtons per meter width.

Example 2

Alternate guarded composite sheets can be used in the present invention.A 100 micrometer thick poly(ethylene terephthalate) (PET) carriersubstrate having an antistatic backing layer (backside) is coated on itsfront surface with a layer promoting adhesion to PVA film comprisingCELVOL 205 PVA (polyvinyl alcohol having a degree of hydrolysis of about88-89%, available from Celanese Corp.) having a dry coating weight ofabout 750 mg/m², and NEOREZ R-600 (from NeoResins Inc.) having a coatingweight of about 250 mg/m². The dried layer is then overcoated with a tielayer comprising poly(ethyl methacrylate-co-methacrylic acid) (acidnumber 130) having a dry coating weight of about 1000 mg/m². The tielayer is overcoated with a triacetyl cellulose (TAC) formulationcomprising three layers: a surface layer comprising CA-438-80S(triacetyl cellulose from Eastman Chemical) having a dry coating weightof about 2080 mg/m², dihexyl cyclohexane dicarboxylate having a drycoating weight of about 208 mg/m², and SURFLON S-8405-S50 (a fluorinatedsurfactant from Semi Chemical Co. Ltd) having a dry coating weight ofabout 210 mg/m²; a mid layer comprising CA-438-80S having a dry coatingweight of about 17370 mg/m², SURFLON S-8405-S50 having a dry coatingweight of about 295 mg/m², dihexyl cyclohexane dicarboxylate having adry coating weight of about 1930 mg/m², TINUVIN 8515 UV absorber havinga dry coating weight of about 650 mg/m², and PARSOL 1789 UV absorberhaving a dry coating weight of about 65 mg/m²; a lower layer comprisinga 47.5:47.5:5 mixture CARBOSET 525 (Noveon Inc.), poly(vinylacetate-co-crotonic acid) (Sigma-Aldrich), and trimethyl borate having adry coating weight of about 1000 mg/m². The TAC formulation was appliedwith a multi-slot slide hopper using a mixture of methylene chloride andmethanol as the coating solvent.

This cover sheet is peeled and laminated as follows in which continuousproduction, including lamination, occurs during roll change and themachine line speed is maintained at 3 meters per minute. The unwindtension of the supply roll is held constant at 10 to 150 Newtons permeter width, controlled by its accumulator position. The PET windingroll has a core outer diameter of 152 mm. The PET winders are controlledby its accumulator position; its tension adjusted by accumulator aircylinder pressure. The PET winding tension is 300 to 400 Newtons permeter width at constant tension (alternatively taper tension may beused). A 125 mm diameter roller is used as the peeling station, wherethe PET carrier web touches the peeling roller. The unguarded coversheet between the peeling station and the lamination nip is controlledto a tension of 100 to 500 Newtons per meter width. The lamination nipforce is set to 100 to 500 Newtons per meter width. After peeled, thePET carrier web tension is 300 t0 400 Newtons per meter width.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   10 coating and drying system-   12 moving substrate-   14 dryer-   16 coating apparatus-   18 unwinding station-   20 back-up roller-   22 coated substrate-   24 guarded cover sheet composite-   26 wind-up station-   28 coating supply vessel-   30 coating supply vessel-   32 coating supply vessel-   34 coating supply vessel-   36 pumps-   38 pumps-   40 pumps-   42 pumps-   44 conduits-   46 conduits-   48 conduits-   50 conduits-   52 discharge device-   54 polar charge assist device-   56 opposing roller-   58 opposing roller-   60 preformed strippable protection layer-   62 unwinding station-   64 wind-up station-   66 drying section    Parts List—Continued-   68 drying section-   70 drying section-   72 drying section-   74 drying section-   76 drying section-   78 drying section-   80 drying section-   82 drying section-   92 front section-   94 second section-   96 third section-   98 fourth section-   100 back plate-   102 inlet-   104 metering slot-   106 pump-   108 lowermost layer-   110 inlet-   112 metering slot-   114 pump-   116 layer-   118 inlet-   120 metering slot-   122 pump-   124 layer-   126 inlet-   128 metering slot-   130 pump-   132 layer    Parts List—Continued-   134 inclined slide surface-   136 coating lip-   138 2^(nd) inclined slide surface-   140 3^(rd) inclined slide surface-   142 4^(th) inclined slide surface-   144 back land surface-   146 coating bead-   151 guarded cover sheet composite (composite sheet)-   153 guarded cover sheet composite (composite sheet)-   159 guarded cover sheet composite-   162 lowermost layer-   164 intermediate layer-   166 intermediate layer-   168 uppermost layer-   170 a, b carrier substrate-   171 cover sheet-   173 cover sheet-   174 lowermost layer-   176 intermediate layer-   178 intermediate layer-   179 cover sheet-   180 uppermost layer-   182 carrier substrate-   184 release layer-   186 lowermost layer-   188 intermediate layer-   189 cover sheet-   190 uppermost layer-   200 a, b supply roll    Parts List—Continued-   201 a, b fresh supply roll-   202 web (PVA-dichroic film)-   203 a, b spindle-   205 downstream lamination pinch rollers-   206 lamination pinch rollers-   207 downstream lamination pinch roller-   208 lamination pinch rollers-   209 downstream bowed roller-   210 a, b carrier winder-   211 conveyance roller-   212 a, b peeling roller-   213 a, b empty core-   214 a, b bowed (bending) roller-   215 a, b load cell roller-   222 fresh web-   223 a, b winding spindle-   216 a, b double-sided splicing means-   217 a, b accumulator rollers-   218 a, b accumulator-   219 a, b clamp-   220 a, b drive roller-   221 a, b frames-   222 fresh web-   224 fresh carrier sheet-   226 fresh adhesive layer-   228 fresh cover sheet-   232 expiring web-   234 pre-peeled cover sheet portion-   236 pre-peeled carrier web portion    Parts List—Continued-   238 a, b single-sided tape-   240 a, b backing support layer-   242 a, b glue-containing layer-   244 expiring cover sheet-   246 expiring carrier sheet-   247 expiring adhesive layer-   250 polarizer plate web/sheet-   252 rear polarizer plate in display-   254 front polarizer plate in display-   256 stationary knife-   258 knife edge-   260 LCD cell-   261 layer promoting adhesion to PVA-   262 tie layer-   264 low birefringence protective polymer film-   266 barrier layer-   268 antiglare layer-   272 viewing-angle compensation layer-   274 a, b double-sided tape-   276 a, b first feedback signal-   278 a, b second feedback signal-   280 roller-   281 roller-   282 supplemental clamp-   284 post-lamination accumulator-   286 feedback signal to carrier winder-   290 first control block-   292 line speed reference-   294 second control block    Parts List—Continued-   297 master drive-   298 reference tension-   300 tension error-   302 tension loop software-   304 speed trim-   305 a, b, c integrator-   306 a, b speed loop software-   307 a, b speed error-   308 a, b current loop-   310 a, b motor-   312 a, b gear box-   314 feedback signal

1. A method of forming a polarizer plate comprising providing at leastone guarded cover sheet composite comprising a carrier substrate and acover sheet, the cover sheet comprising a layer promoting adhesion topolyvinyl alcohol and a low birefringence polymer film, providing apolarizing film, and bringing the cover sheet into contact with thepolarizing film such that the layer promoting adhesion to polyvinylalcohol in the cover sheet is in contact with the polarizing filmthereby producing a polarizer plate composite sheet comprising a coversheet and a polarizing film adhesively joined by the layer promotingadhesion to polyvinyl alcohol, wherein the carrier substrate is peeledfrom the cover sheet prior to bringing the cover sheet into contact withthe polarizing film, and wherein the method further comprises conveyingat least one of the cover sheet, guarded cover sheet composite, andpolarizer film through an accumulator to allow continuous production ofthe composite polarizer film.
 2. The method of claim 1 wherein theguarded cover sheet composite is conveyed through an accumulator that ispositioned between a supply roll for the guarded cover sheet compositeand a peeling station.
 3. The method of claim 2 wherein the accumulatoris positioned between a tension isolating means, located after adouble-sided splicing means, and a drive roller.
 4. The method of claim2 wherein the carrier substrate after being peeled is conveyed through adifferent accumulator positioned between the peeling station and acarrier web winder.
 5. The method of claim 1 wherein the compositepolarizer film is conveyed through an accumulator positioned between alaminator nip and a winder for the composite polarizer film.
 6. Themethod of claim 5 wherein said accumulator is positioned between thelaminator nip and a dryer for the composite polarizer film.
 7. Themethod of claim 2 wherein a double-sided splicing means is locatedbetween the supply roll and a peeling station.
 8. The method of claim 1wherein the method comprises a peeling station and a lamination stationthat is also continuous during steady state operation.
 9. The method ofclaim 1 wherein the layer promoting adhesion to polyvinyl alcohol isadjacent to the carrier substrate.
 10. The method of claim 1 wherein thecarrier substrate comprises polyester.
 11. The method of claim 1 whereinthe low birefringence polymer film comprises cellulose ester.
 12. Themethod of claim 1 wherein the cover sheet comprises at least onefunctional/auxiliary layer.
 13. The method of claim 12 wherein thefunctional layer is a moisture barrier layer, antistatic layer,compensation layer, hard coat, antiglare, or anti-reflection layer. 14.The method of claim 13 wherein the hard coat, antiglare, oranti-reflection layers are on the side of the low birefringence polymerfilm opposite to the layer promoting adhesion to polyvinyl alcoholfilms.
 15. The method of claim 7 wherein the double-sided splicing meansproduces a butt splice or an overlap splice.
 16. The method of claim 1further comprising a first and a second means for isolating tension inthe accumulator during a splicing operation.
 17. The method of claim 16wherein the first and the second means for isolating tension comprises,respectively, a clamp and a tension isolation drive.
 18. The method ofclaim 1 wherein the cover sheet is peeled by a single roller, acontacting nip comprised of two rollers, a knife edge, or a peeling rod.19. The method of claim 1 further comprising conveying the cover sheetthrough a means for spreading a web.
 20. The method of claim 19 whereinthe means for spreading a web is a bowed or bending roller, concaveroller, or flex roller.
 21. The method of claim 1 wherein the coversheet has a thickness less than about 40 μm.
 22. The method of claim 1further comprising drying the composite polarizer film by allowingmoisture to be removed therefrom with or without application of heat.23. The method of claim 1 wherein glue is applied to the cover sheet orpolarizing film prior to contact with each other.
 24. The method ofclaim 1 wherein glue is applied to the cover sheet or polarizing film byspraying, dripping, roll coating, hopper coating, or knife coating. 25.The method of claim 23 wherein the glue is applied to the cover sheetafter it is conveyed past a means for spreading a web.
 26. The method ofclaim 1 wherein the guarded cover sheet composite is heated prior tobeing peeled.
 27. The method of claim 1 wherein electrostatic charge isremoved from the cover sheet after being peeled.
 28. The method of claim1 wherein electrostatic charge is removed from the guarded cover sheetcomposite in proximity to a supply roll for providing the guarded coversheet composite.
 29. The method of claim 1 wherein the polarizer platecomposite sheet after drying is wound on a winding spindle.
 30. Themethod of claim 1, further comprising, after the carrier substrate ispeeled from the cover sheet, winding each carrier substrate onto awinding spindle.
 31. The method of claim 2 wherein the carriersubstrate, after having been peeled from the cover sheet, is conveyedthrough an accumulator positioned between two tension-controllerdevices.
 32. The method of claim 1 further comprising disposing of thecarrier substrate, after having been peeled from the cover sheet, to awaste chopper for recycling.
 33. The method of claim 1 wherein the layerpromoting adhesion to polyvinyl alcohol films comprises a hydrophilicpolymer.
 34. The method of claim 33 wherein the hydrophilic polymercomprises polyvinyl alcohol.
 35. The method of claim 1 wherein the layerpromoting adhesion to polyvinyl alcohol films further comprises acrosslinking compound.
 36. The method of claim 1 wherein the lowbirefringence protective polymer film comprises a polycarbonate or acyclic polyolefin.
 37. A method of forming a polarizing plate comprisingthe following steps: (a) supplying a first and a second web from a firstand a second supply roll, respectively, each of the first and the secondweb comprising a guarded cover sheet having a carrier substrate and aprotective cover sheet, the protective cover sheet comprising a lowbirefringence polymer protective film and a layer promoting adhesion topolyvinyl alcohol; (b) conveying each web in proximity to a means fordouble-sided splicing each web with a fresh web; when each of the firstor the second web is near to expiring, which can occur independently;(c) conveying each web through an accumulator, located either prior to apeeling station of step (d) or following a lamination of step (g) belowsuch that supply of a polarizer sheet to the dryer in step (h) issubstantially constant; (d) for each web, removing the carrier substratefrom the protective cover sheet at a peeling station to produce (i) anunguarded web comprising the protective cover sheet and (ii) a carrierweb comprising the carrier substrate; (e) optionally conveying eachunguarded web over a means for spreading the web; (f) bringing eachunguarded web, either simultaneously or sequentially, into contact witha polarizing web comprising a dichroic PVA film such that each layerpromoting adhesion to polyvinyl alcohol, in each unguarded web, iscontacted with the dichroic PVA film, wherein pressure is applied as thePVA dichroic film and protective cover sheets are brought into contact,thereby forming a polarizer plate web; and (g) drying the polarizerplate web.
 38. A method of forming a polarizing plate comprising thefollowing steps (a) supplying a web from a supply roll, the webcomprising a guarded cover sheet having a carrier substrate and aprotective cover sheet, the protective cover sheet comprising a lowbirefringence polymer protective film and a layer promoting adhesion topolyvinyl alcohol; (b) optionally conveying the web in proximity to ameans for double-sided splicing the web with a fresh web when the web isnear to expiring; (c) optionally conveying the web through a driveroller; (d) removing the carrier substrate from the protective coversheet at a peeling station to produce (i) an unguarded web comprisingthe protective cover sheet, and (ii) a carrier web comprising thecarrier substrate; (e) optionally conveying each unguarded web over ameans for spreading the web; (f) bringing the unguarded web into contactwith a polarizing web comprising a dichroic PVA film such that the layerpromoting adhesion to polyvinyl alcohol, in the unguarded web, iscontacted with the dichroic PVA film, wherein pressure is applied as thePVA dichroic film and cover sheet are brought into contact forming apolarizing plate composite sheet; (g) optionally contacting, eithersimultaneously or sequentially with step (f), the polarizing web with asecond unguarded web on the opposite side of the polarizing web to forma polarizer plate web; and (h) drying the polarizer plate web.